The underhook controls the hip

"The side of the body with the underhook controls the hip on that side. Hip control determines directional movement."

The underhook — the arm threaded under the opponent’s arm to grip the back or shoulder — is not simply a wrestling grip. It is a mechanism for controlling the hip on the same side as the underhook. When you have the underhook on your right, you control the opponent’s right hip. When you have it on your left, you control the left hip. The hip you control determines the direction the opponent’s body can move.

This works because the underhook connects the upper body to the torso, and the torso connects to the hip. The underhook allows the player who holds it to steer the opponent’s structure. The opponent’s hip on the underhook side cannot move away freely — it is anchored by the connection through the arm and shoulder. The opponent’s body is thus steered in the direction the underhook dictates.

Hip control is directional movement control. Whoever controls the hips controls where the body goes. In grappling, the hips are the most important segment of the body to control — they are the structural center. The underhook is the most direct mechanism for establishing that control from a clinch or tie-up position.

Across the system, this principle expresses most cleanly in the following techniques:

Half guard underhook coming up: The bottom player’s underhook locks onto the top player’s hip on that side, which is what allows the come-up to a single leg or back take. The hip cannot rotate away from the underhook without the underhook coming with it. Without the underhook, the same come-up motion meets a rotating top hip and stalls.

Butterfly sweep: The lift only sweeps because the bottom player’s underhook is locked to the same-side hip the butterfly hook is elevating. Underhook and hook are working on the same hip from two angles. A butterfly hook without the underhook lifts a hip the opponent can simply walk away from.

Arm drag (standing or seated): The drag pulls the opponent past the attacker by yanking the inside-shoulder arm across, but the drag only delivers the back because the same motion drives the attacker’s near shoulder under the opponent’s arm — establishing the underhook and so controlling the hip. If the underhook is not landed, the opponent’s hip rotates back to face and the drag yields nothing.

Over-under clinch (no-gi): Whoever’s underhook side is deeper controls that side’s hip and dictates the direction of the clinch. The over-under deadlock often resolves through a shrug or a duck-under because one player succeeds in deepening the underhook to the point of driving the opponent’s hip backward.

Side control with underhook: The far-side underhook from side control prevents the bottom player from rotating their far hip up into the recovery shrimp. The hip on the underhook side is the hip that needs to move to escape; the underhook removes that movement.