Quick answer: On face-on video, fewer than two knuckles of the lead hand are visible at address; the thumb-index V points at the chin or the lead shoulder.
What it does to the ball
Open face at impact: the ball starts right of the target (the face governs roughly 75-85% of the start line) and falls off in a fade or slice; high, short flight.
Root causes (by frequency)
Placing the grip in the palm instead of the fingers
Copying a generic neutral grip without adjusting it to your own anatomy
Over-weakening the grip out of fear of an old hook
How to fix it
Rotate the lead hand until you see 2-3 knuckles at address, with the V pointing at your trail shoulder
Drills that fix it
The knuckle mirror · 10 min · 15 reps
Install a neutral-to-strong grip that squares the face In front of a mirror, take your grip and check that you can see 2-3 knuckles of the lead hand. Rebuild the grip from scratch, closing your eyes at the end to memorize the feel. Finish with 50% swings, checking the grip before each one.
The glove line · 10 min · 12 reps
Place the club in the fingers, not the palm Draw a diagonal line on your glove from the base of the pinky to the middle of the index finger. Rest the grip exactly on that line before closing the hand. Hit short shots, checking the line on every regrip.
Frequently asked questions
What is "Weak grip (open face)" in the golf swing?
On face-on video, fewer than two knuckles of the lead hand are visible at address; the thumb-index V points at the chin or the lead shoulder.
What does weak grip (open face) do to the ball?
Open face at impact: the ball starts right of the target (the face governs roughly 75-85% of the start line) and falls off in a fade or slice; high, short flight.
How do you fix weak grip (open face)?
Rotate the lead hand until you see 2-3 knuckles at address, with the V pointing at your trail shoulder
Sources
Ben Hogan — Five Lessons: The Modern Fundamentals of Golf