Quick answer: On video, the lead wrist breaks down after impact and the arms-shoulders triangle changes shape during the stroke.
What it does to the ball
The face rotates at impact: the ball starts off line even with a good read, and distance becomes unpredictable.
Root causes (by frequency)
Hitting with the hands instead of rocking the shoulders
Grip pressure that varies during the stroke
How to fix it
The shoulders-arms triangle moves as one piece: lead wrist flat and firm all the way to the finish
Drills that fix it
Stick under the watch · 10 min · 20 reps
Lock out the lead-wrist breakdown in the putt Slide a short stick or a long pencil under your watch strap, flat against the back of the lead wrist. Hit 2-3 m putts: if the wrist breaks down, the stick lifts off the forearm and you feel it instantly.
The intact triangle · 10 min · 20 reps
Rock shoulders and arms as one piece Form the arms-shoulders triangle at address and make strokes feeling that the chest is the engine. The elbows keep their address flex from start to finish. Record from the front and check that the triangle never changes shape.
Frequently asked questions
What is "Wrist breakdown in the putting stroke" in the golf swing?
On video, the lead wrist breaks down after impact and the arms-shoulders triangle changes shape during the stroke.
What does wrist breakdown in the putting stroke do to the ball?
The face rotates at impact: the ball starts off line even with a good read, and distance becomes unpredictable.
How do you fix wrist breakdown in the putting stroke?
The shoulders-arms triangle moves as one piece: lead wrist flat and firm all the way to the finish
Sources
Dave Stockton (con Matthew Rudy) — Unconscious Putting