When a guy or girl curls his or her middle finger and surprisingly tries to hook a girl in the vagina.

Fish hook- a sexual manoevre in which one person uses a finger or a thumb to form a crudely improvised hook inside the orrifice of another, followed by exerting an outward, stretching pressure on said orrifice.

While performing intercourse "doggie-style" this maneuver involves the pitcher inserting his curled finger into the catcher's mouth and pulling back to degrade the subject or cause pain.
A "double fish hook" involves the same maneuver using both fingers.
A "rusty fish hook" involves fingering the ass of the catcher prior to insertion of the finger(s) into the mouth.

Putting your index and middle finger into a females mouth during intercourse. Simulating when a fish gets hooked on a Fish Hook. A well executed Fish Hook should prompt a startled look from the fish hooked.

Putting a finger in someones mouth while trying to hurt them and then pulling as hard as you can to rip their cheek. This resembles a fish being hooked so is called the fish hook.
As seen in The Gangs Of New York

using your index finger to pull down a girls shirt in order to see her boobs.

When one grabs a women with a claw like hand in the private area.