Quick answer: The bunker backswing is long and the club visibly slows down as it reaches the sand; the follow-through is short or nonexistent.
What it does to the ball
The sand steals all the energy from a braking club: the ball stays in the bunker or dies at the lip.
Root causes (by frequency)
A backswing too long for a shot that scares you
Fear of flying the green with that much club
Not trusting that the sand absorbs a big part of the speed
How to fix it
The sand brakes, you accelerate: a full, high follow-through on every bunker shot — distance is set by the length of the backswing, never by braking
Drills that fix it
The splash with no ball · 15 min · 20 reps
Accelerate through the sand with a full follow-through Without a ball, make bunker swings throwing a handful of sand over the lip onto the green: listen for the 'pfff' of the splash and finish with the hands high on every one. The flying sand is your ball. Once you throw 10 splashes in a row with a high finish, put a ball in the middle of the sand and repeat exactly the same motion.
Frequently asked questions
What is "Deceleration in the sand" in the golf swing?
The bunker backswing is long and the club visibly slows down as it reaches the sand; the follow-through is short or nonexistent.
What does deceleration in the sand do to the ball?
The sand steals all the energy from a braking club: the ball stays in the bunker or dies at the lip.
How do you fix deceleration in the sand?
The sand brakes, you accelerate: a full, high follow-through on every bunker shot — distance is set by the length of the backswing, never by braking
Sources
Dave Pelz — Dave Pelz's Short Game Bible
Harvey Penick — Harvey Penick's Little Red Book
PGA of America — PGA Teaching Manual: The Art and Science of Golf Instruction