Quick answer: On video, the chipping backswing is longer than the follow-through and the clubhead visibly slows down before reaching the ball.
What it does to the ball
Alternating fats and thins: deceleration leaves the clubhead with no direction and the low point behind the ball.
Root causes (by frequency)
A backswing too long for the distance, forcing you to brake
Fear of airmailing the green after a previous fat chip
How to fix it
Follow-through equal to or longer than the backswing: shorten the backswing and accelerate smoothly through the ball
Drills that fix it
Short back, long through · 10 min · 20 reps
Install acceleration in the chip Set two mental lengths: backswing to the shin and follow-through to the knee. Hit chips where the follow-through always beats the backswing. If the ball goes long, shorten the backswing — never brake the club.
Lead-arm-only chip · 10 min · 12 reps
Feel smooth acceleration without the dominant hand Hit short chips with the lead arm only, trail hand behind your back. It's nearly impossible to decelerate without losing control of the club: the motion turns pendular by itself. Then put both hands back on and replicate the feeling.
Frequently asked questions
What is "Deceleration in the chip" in the golf swing?
On video, the chipping backswing is longer than the follow-through and the clubhead visibly slows down before reaching the ball.
What does deceleration in the chip do to the ball?
Alternating fats and thins: deceleration leaves the clubhead with no direction and the low point behind the ball.
How do you fix deceleration in the chip?
Follow-through equal to or longer than the backswing: shorten the backswing and accelerate smoothly through the ball
Sources
Dave Pelz — Dave Pelz's Short Game Bible
Stan Utley (con Matthew Rudy) — The Art of the Short Game