Quick answer: Face-on: 4 knuckles of the lead hand are visible and the trail hand is rotated well under the grip; the face arrives shut at the top.
What it does to the ball
Face closed relative to the path: the ball starts left with a hook and a low flight; with the driver the curve gets amplified.
Root causes (by frequency)
Chasing distance by closing the face with the hands
Overcorrecting an old slice
Trail hand rotated too far under the club
How to fix it
Return to a neutral grip: 2-3 knuckles visible and both Vs parallel, pointing at the trail shoulder
Drills that fix it
Back to neutral · 15 min · 20 reps
Soften an overly strong grip without losing confidence In front of the mirror, rotate the lead hand until only 2-3 knuckles show, and point the trail-hand V at your trail shoulder. Hit balls at 60% with an 8-iron, accepting that they'll start right at first. Move up a club only once the flight straightens out.
Frequently asked questions
What is "Overly strong grip (closed face)" in the golf swing?
Face-on: 4 knuckles of the lead hand are visible and the trail hand is rotated well under the grip; the face arrives shut at the top.
What does overly strong grip (closed face) do to the ball?
Face closed relative to the path: the ball starts left with a hook and a low flight; with the driver the curve gets amplified.
How do you fix overly strong grip (closed face)?
Return to a neutral grip: 2-3 knuckles visible and both Vs parallel, pointing at the trail shoulder
Sources
Ben Hogan — Five Lessons: The Modern Fundamentals of Golf
PGA of America — PGA Teaching Manual: The Art and Science of Golf Instruction