Quick answer: Down-the-line: halfway back the hands are well inside and the shaft points inside the ball line; the club gets stuck behind the body.
What it does to the ball
A strongly in-to-out path on the way down: blocks to the right, or sudden hooks when the hands overcorrect.
Root causes (by frequency)
An inside takeaway with early forearm rotation
A very flat, level shoulder turn with no tilt over the plane
How to fix it
Halfway back the shaft should point at the ball line: check that frame on down-the-line video every few swings
Drills that fix it
Halfway-back snapshot · 15 min · 15 reps
Fix a too-flat backswing with visual feedback Record down-the-line and pause the video when the shaft is horizontal going back: it should point parallel to the ball line, not inside it. Alternate one slow-motion rehearsal and one real shot. Review the video every 5 swings.
Frequently asked questions
What is "Backswing plane too flat" in the golf swing?
Down-the-line: halfway back the hands are well inside and the shaft points inside the ball line; the club gets stuck behind the body.
What does backswing plane too flat do to the ball?
A strongly in-to-out path on the way down: blocks to the right, or sudden hooks when the hands overcorrect.
How do you fix backswing plane too flat?
Halfway back the shaft should point at the ball line: check that frame on down-the-line video every few swings
Sources
Ben Hogan — Five Lessons: The Modern Fundamentals of Golf