Quick answer: The club contacts the ball clean (or enters the sand right at it) instead of entering a couple of inches behind; the ball rockets low into the lip or across the whole green.
What it does to the ball
With no cushion of sand the face contacts the ball directly: a thin screamer into the lip, or a ball that crosses the green with no height or spin.
Root causes (by frequency)
The instinct to strike the ball clean, fairway-style
Fear of leaving it in the bunker after a previous short one
Ball too far back in the bunker stance
How to fix it
In the bunker the club NEVER touches the ball: enter the sand a couple of inches behind it and let the cushion of sand push the ball out
Drills that fix it
The line in the sand · 15 min · 15 reps
Enter the sand behind the ball every time, never at it Draw a long line in the sand with your finger, perpendicular to the target. Without a ball, make swings trying to enter the sand right at the line, moving along it after each swing. Once you nail 5 in a row, place balls a couple of inches ahead of the line and repeat: the club enters at the line and the ball comes out on its cushion of sand.
The bill under the ball · 15 min · 15 reps
Take a thin, consistent cushion of sand, letting the sole do the work Picture the ball in the middle of a banknote lying on the sand. Your job is not to hit the ball: it's to remove the whole bill — the club enters where the bill starts and exits where it ends, shallow and accelerating. Draw the rectangle in the sand the first few times as a visual reference.
Frequently asked questions
What is "Hitting the ball before the sand" in the golf swing?
The club contacts the ball clean (or enters the sand right at it) instead of entering a couple of inches behind; the ball rockets low into the lip or across the whole green.
What does hitting the ball before the sand do to the ball?
With no cushion of sand the face contacts the ball directly: a thin screamer into the lip, or a ball that crosses the green with no height or spin.
How do you fix hitting the ball before the sand?
In the bunker the club NEVER touches the ball: enter the sand a couple of inches behind it and let the cushion of sand push the ball out
Sources
Dave Pelz — Dave Pelz's Short Game Bible
Stan Utley (con Matthew Rudy) — The Art of the Short Game