The Head Power Locks are designed to ensure a secure fit between the heel cup of the fin and the foot. They also be worn inside the fin to reduce blisters caused by some fins. They are manufactured of chlorine resistant rubber to prevent unnecessary damage.
What size should I buy?
Small - Fin Sizes 3, 4 and 5
Medium - Fin Sizes 5, 6 and 7
Large - Fin Sizes 8, 9, 9 1/2 and 10 1/2