"Keep your left arm straight" might be the most over-obeyed instruction in golf. Golfers lock the arm rigid, tension creeps up through the shoulders, and the swing gets shorter and slower — the exact opposites of the goal. So let's replace the cue: firm, never locked.
Here's why the lead arm earns so much attention in the first place. It sets your swing's radius. A comfortably extended lead arm at the top keeps the clubhead traveling on a wide arc — and a wide arc gives the club more distance to gather speed on the way down. When the arm folds at the elbow up there, the arc contracts. The swing gets shorter without getting quicker, and the fold has to un-fold on the way down — one more moving part demanding perfect timing before impact.
The usual cause isn't a weak arm — it's an ambitious one. Most collapses happen because the arms keep traveling after the chest stops turning. Your shoulders reach the end of their comfortable coil, the hands keep chasing a longer backswing, and the elbow buckles to grant the extra length. It feels like a bigger swing. It's actually a shorter arc wearing a longer costume: width traded for length, and width is where the power lives.
How straight your lead arm stays at the top is one of the checkpoints CoachVision reads from an ordinary phone clip — worth knowing, because a swing that feels compact often measures wide, and a swing that feels huge often measures folded.
The drill — L-to-L Swings
Take a wedge. Swing back until your lead arm points at 9 o'clock — parallel to the ground — with your wrists hinged so the arm and club form a capital "L." Swing through to the mirror image on the other side, trail arm at 3 o'clock, wrists rehinged.
Notice the assignment hiding in that first "L": the lead arm stays comfortably extended while the wrists do the folding. That's the entire division of labor in a sound backswing — wrists hinge, arm stays long. The drill also ends your backswing where your structure ends, teaching arms and chest to stop together instead of the arms freelancing past the turn.
Do 15 reps with a wedge, unhurried. When the 9 o'clock "L" feels natural, let the swings grow — and watch for the moment the old fold tries to sneak back in, usually right when you start chasing distance again.
A shorter swing with a wide arc will outhit a long one with a folded arm — and it'll do it with timing to spare. Firm, never locked, and let the width do the work.