Is going past parallel a fault on its own?

No. It records where the shaft finished, and the Titleist Performance Institute treats it as a symptom to trace rather than an error to delete. It matters when the extra length came from a joint that must unfold before the club changes direction, because the unfolding then has to be timed.

Will a shorter backswing make me strike it better?

Bulbulian's golfers shortened by 46.5 degrees and their mean strike deviation on the clubface barely moved, 19.0 mm against 19.3 mm. Adam Young's position is that shorter swings are generally more controlled and give up some distance for it. The distance half of that trade has been measured. The accuracy half has not.

Does a longer backswing produce more clubhead speed?

It gives you more room to accelerate over, which is why Adam Young calls a longer swing more power potential rather than more power. Dale and Brumitt's thirteen golfers lost about 2 m/s when they shortened up. The potential is only collectable when the extra range came from turning.

My lead elbow folds at the top. Should I hold it straight?

A straight arm at the top is a consequence rather than a brace. If the elbow folds because the ribcage stopped rotating, holding it rigid gives you a shorter swing with the same restriction underneath. The Titleist Performance Institute puts the cue at the trunk instead: keep the twelve inches or so between lower rib and pubic bone that you had at address.

How do I find parallel on my own video?

Film down the line and stop on the frame where the club stops going back. Parallel is the shaft horizontal there. Camera height tilts that line, so a phone on the ground and a phone at chest height disagree about the same swing. Compare length only against your own earlier clips.

What goes wrong, and why

Past parallel: where the extra length comes from

By the end you will know which part of your body actually sent the club past parallel, what shortening a backswing has been measured to cost, and how to tell a long swing from a loose one.

A golfer in a white skirt and cap swinging on a fairway, the club well past the shoulders. David Wilson from Oak Park, Illinois, USA, CC BY 2.0

Parallel describes the shaft, not the golfer

The reference position at the top is the shaft horizontal, pointing roughly along your target line. Past parallel means it has dipped below that, so the clubhead aims down at the ground behind you. The Titleist Performance Institute defines an overswing in exactly those terms. Where the shaft points once it arrives is a separate question with its own guide.

All that records is where a metal tube finished. It says nothing about the joint that put it there. Two golfers can reach the identical picture with nothing in common: one turned a long way and kept the arm structure they began with, the other stopped turning early and let an elbow and two wrists finish the job.

Seven ways to get longer, and two of them are turn

Two of sevenBackswing-lengthening mechanisms that are body turn, in Adam Young's list of seven

Adam Young sets out the mechanisms that lengthen a backswing, and there are seven: a bigger shoulder turn, collapsing the arms, moving the lead upper arm further across the chest, adding vertical arm movement, a bigger hip turn with the lead foot coming off the ground, more wrist hinge, and changing how much the hips and spine extend.

Two of those seven are the body turning. The other five are arms, wrists and spine reorganising around a turn that has already run out. Saying you have a long backswing therefore says nothing about what you did, because the word names an outcome five different joints can manufacture alone.

The Titleist Performance Institute gives two bodily reasons for the habit. One is hypermobility: plenty of range, not enough strength to govern it. The other is counterintuitive. A thoracic spine with no rotation in it leaves the arms and shoulders to cover the shortfall, so the golfer whose ribcage turns least can be the one whose club travels furthest.

What John Daly's extra length is made of

John Daly's shaft crosses parallel and, at full stretch, wraps behind him toward vertical. He won the PGA Championship in 1991 and the Open at St Andrews in 1995 carrying it. Whatever past parallel is, a position that hands you two major championships is not a fault in itself.

More useful than the picture is the claim printed beside it: that Daly's shoulder turn is no larger than the tour average, and the length comes from hands and wrists unusually free at the top. Read that as an instructor describing a video rather than a measurement. Either way, two golfers matching that silhouette may have loaded entirely different joints to reach it.

Hear it

Three clicks: takeaway, top, impact. Start at the tour beat and move to whichever one you can actually swing to.

The two studies that shortened a backswing

≈2 m/s, ≈10 mClubhead speed and 7-iron distance lost by thirteen golfers shortening the backswing

Bulbulian, Ball and Seaman put seven golfers through a full swing and a deliberately shortened one, filming on high-speed video and reading clubhead velocity off a swing speed indicator. Cutting the backswing by 46.5 degrees, give or take 24.7, left clubhead velocity at 31.2 m/s against 33.9 m/s, which the paper reports as not significantly reduced.

That phrase carries more weight than it can hold. The gap is 2.7 m/s, near enough six miles an hour, and it missed the significance threshold in a group of seven. A null result from a sample that small is not evidence that shortening your swing is free.

Dale and Brumitt ran the comparison on thirteen golfers hitting ten swings each with their own 7-iron. The shortened version cost roughly 2 m/s of clubhead velocity and roughly 10 m of shot distance, both significant. Vertebral compression force, normalised to body weight, fell from 7.6 to 7.0.

The trade is real and it is modest: about eleven yards of 7-iron for a lighter load through the lumbar spine. What neither study found was a cleaner strike. Bulbulian recorded where the ball met the clubface, and mean deviation was 19.0 mm full against 19.3 mm short.

Telling a long backswing from a loose one

Your lead arm is straight at address. Find the frame where the club changes direction and measure that elbow again. If the angle survived and the shaft went past horizontal anyway, the length came from rotation and from the arm travelling across your chest. If the elbow folded in between, that fold has to be undone before the clubhead can start down.

GOLFTEC, whose OptiMotion system measures this joint, puts the difference as a tendency rather than a threshold: better players tend to carry less lead elbow flex at the top, less experienced golfers more. No figure in degrees is published with it, so treat it as a direction of travel and not a target angle.

Do it on three clips rather than one. An elbow that folds in a single video and holds in the other two is more likely the camera than the swing. Pebble Swing reads joint angles out of the frames of an uploaded video, which is the measurement this comparison wants. What one lens cannot settle is how much of the dip is wrist rather than elbow.

Questions

Is going past parallel a fault on its own?: No. It records where the shaft finished, and the Titleist Performance Institute treats it as a symptom to trace rather than an error to delete. It matters when the extra length came from a joint that must unfold before the club changes direction, because the unfolding then has to be timed.
Will a shorter backswing make me strike it better?: Bulbulian's golfers shortened by 46.5 degrees and their mean strike deviation on the clubface barely moved, 19.0 mm against 19.3 mm. Adam Young's position is that shorter swings are generally more controlled and give up some distance for it. The distance half of that trade has been measured. The accuracy half has not.
Does a longer backswing produce more clubhead speed?: It gives you more room to accelerate over, which is why Adam Young calls a longer swing more power potential rather than more power. Dale and Brumitt's thirteen golfers lost about 2 m/s when they shortened up. The potential is only collectable when the extra range came from turning.
My lead elbow folds at the top. Should I hold it straight?: A straight arm at the top is a consequence rather than a brace. If the elbow folds because the ribcage stopped rotating, holding it rigid gives you a shorter swing with the same restriction underneath. The Titleist Performance Institute puts the cue at the trunk instead: keep the twelve inches or so between lower rib and pubic bone that you had at address.
How do I find parallel on my own video?: Film down the line and stop on the frame where the club stops going back. Parallel is the shaft horizontal there. Camera height tilts that line, so a phone on the ground and a phone at chest height disagree about the same swing. Compare length only against your own earlier clips.

Sources

Every number on this page comes from one of these.