Quick answer: Down-the-line: at the top the trail elbow separates from the side and points backwards instead of at the ground; the shaft goes across the line.
What it does to the ball
It invites an over-the-top downswing: pull-slice, strikes toward the toe and lost compression.
Root causes (by frequency)
An arms-dominated backswing with no connection to the torso
Excessive trail-forearm rotation
How to fix it
Towel under the trail armpit: keep it pinned to the top so the elbow points at the ground
Drills that fix it
The armpit towel · 10 min · 15 reps
Connect the trail arm to the torso and bring the elbow down Tuck a towel or headcover under your trail armpit. Make three-quarter swings without dropping it until after impact. Start with half swings and lengthen only when the towel survives 10 shots in a row.
Frequently asked questions
What is "Flying trail elbow" in the golf swing?
Down-the-line: at the top the trail elbow separates from the side and points backwards instead of at the ground; the shaft goes across the line.
What does flying trail elbow do to the ball?
It invites an over-the-top downswing: pull-slice, strikes toward the toe and lost compression.
How do you fix flying trail elbow?
Towel under the trail armpit: keep it pinned to the top so the elbow points at the ground
Sources
David Leadbetter — The Golf Swing
PGA of America — PGA Teaching Manual: The Art and Science of Golf Instruction