Quick answer: Impact mark toward the heel or the hosel; on video, the hands are farther from the body at impact than they were at address.
What it does to the ball
Off the heel the ball loses speed and tends to fade; off the hosel it rockets right at an angle (the shank).
Root causes (by frequency)
Early extension pushing the hands outward
Weight moving into the toes on the downswing
An over-the-top downswing
How to fix it
Tee gate: one tee outside the toe and one inside the heel — swing the club through without touching the outer one
Drills that fix it
The tee gate · 15 min · 20 reps
Center the strike and eliminate the heel hit Push two tees into the ground forming a gate barely wider than the clubhead: one outside the toe and one inside the heel. Swing the club through without touching either — rehearsals first, then with a ball. Hitting the inside tee betrays hands pushed outward.
Frequently asked questions
What is "Heel strikes (shank risk)" in the golf swing?
Impact mark toward the heel or the hosel; on video, the hands are farther from the body at impact than they were at address.
What does heel strikes (shank risk) do to the ball?
Off the heel the ball loses speed and tends to fade; off the hosel it rockets right at an angle (the shank).
How do you fix heel strikes (shank risk)?
Tee gate: one tee outside the toe and one inside the heel — swing the club through without touching the outer one
Sources
Adam Young — The Practice Manual
PGA of America — PGA Teaching Manual: The Art and Science of Golf Instruction