Quick answer: The ball starts to one side of the chosen line even on short, straight putts; a tee gate barely wider than the putter head exposes the twist on the first try.
What it does to the ball
The face governs the putt's start direction: with the face slightly open or closed, well-read 2 m putts slide past the edges.
Root causes (by frequency)
Active wrists rotating the face during the stroke
Grip pressure that changes right before impact
Trying to steer the ball with the hands instead of trusting the chosen line
How to fix it
Face square to YOUR line through the whole impact: swing the putter through a tee gate barely wider than the head and the ball will start straight on its own
Drills that fix it
The putter gate · 10 min · 20 reps
Keep the face square through impact Push two tees into the green forming a gate barely wider than your putter head. Make strokes swinging the putter through the gate without touching the tees, first without a ball and then with 1-2 m putts. If the face twists or the path bends, you'll hit a tee and know immediately. Eight clean out of ten to close the session.
Frequently asked questions
What is "Putter face opening or closing at impact" in the golf swing?
The ball starts to one side of the chosen line even on short, straight putts; a tee gate barely wider than the putter head exposes the twist on the first try.
What does putter face opening or closing at impact do to the ball?
The face governs the putt's start direction: with the face slightly open or closed, well-read 2 m putts slide past the edges.
How do you fix putter face opening or closing at impact?
Face square to YOUR line through the whole impact: swing the putter through a tee gate barely wider than the head and the ball will start straight on its own
Sources
Dave Pelz — Dave Pelz's Short Game Bible
Dave Stockton (con Matthew Rudy) — Unconscious Putting