Quick answer: Down-the-line: the club goes up outside the hand line with little torso rotation; at the top the hands are very high, above the functional plane.
What it does to the ball
A steep out-to-in downswing: deep divots, pulls and a slice; with the driver, high spin and lost carry.
Root causes (by frequency)
Lifting the club with the arms without turning the torso
Standing too upright at address
How to fix it
Feel the club swing around your body with the shoulder turn: halfway back the shaft points at the ball line
Drills that fix it
Under the stick roof · 15 min · 12 reps
Flatten a backswing that's too steep Lean an alignment stick against a chair, following the club's plane above your trail shoulder. Swing back feeling the club travel just underneath the stick, around your body. Alternate rehearsals and gentle real shots.
Frequently asked questions
What is "Backswing plane too steep" in the golf swing?
Down-the-line: the club goes up outside the hand line with little torso rotation; at the top the hands are very high, above the functional plane.
What does backswing plane too steep do to the ball?
A steep out-to-in downswing: deep divots, pulls and a slice; with the driver, high spin and lost carry.
How do you fix backswing plane too steep?
Feel the club swing around your body with the shoulder turn: halfway back the shaft points at the ball line
Sources
David Leadbetter — The Golf Swing
Dr. Young-Hoo Kwon, Texas Woman's University — Golf Biomechanics Instructor Program (secuencia cinemática, plano funcional)