Quick answer: At address the sand wedge face is square or closed, as for a normal grass shot; the club buries itself when it enters the sand.
What it does to the ball
The leading edge digs instead of gliding: fat shots that stay in, or thins that rocket out when the hands try to rescue it.
Root causes (by frequency)
The feeling that an open face will send the ball right
Opening the face by twisting the hands AFTER taking the grip (the opening is lost during the swing)
Treating the sand wedge like just another iron
How to fix it
Open the face BEFORE taking your grip: rotate the club until the face looks at the sky, only then place your hands, aiming the body a little left to compensate
Drills that fix it
Face to the sky · 10 min · 12 reps
Install the routine of opening the face before taking the grip Outside the bunker, rotate the sand wedge until the face looks at the sky (you could rest a tee on it without it falling) and ONLY then place your hands. Step into the bunker with that routine: open face first, grip second, body slightly left. Hit short shots checking that the ball comes out high and soft.
Frequently asked questions
What is "Face not opened in the sand" in the golf swing?
At address the sand wedge face is square or closed, as for a normal grass shot; the club buries itself when it enters the sand.
What does face not opened in the sand do to the ball?
The leading edge digs instead of gliding: fat shots that stay in, or thins that rocket out when the hands try to rescue it.
How do you fix face not opened in the sand?
Open the face BEFORE taking your grip: rotate the club until the face looks at the sky, only then place your hands, aiming the body a little left to compensate
Sources
Dave Pelz — Dave Pelz's Short Game Bible
PGA of America — PGA Teaching Manual: The Art and Science of Golf Instruction