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Wrapping your arm over the opponent's arm to trap it, from guard (for triangles and sweeps) or standing (as a whizzer).
Also called overhooks, whizzer from guard, overhook guard.
3 resources
Stephan Kesting YouTube
Jon Thomas on hitting the triangle from the overhook in closed guard, including the anti-stack motion that keeps your hips on the floor when they drive in. From his Grapplearts closed guard course.
A full tour of butterfly guard for gi and no-gi: underhook and overhook grips, the classic hook sweep, entries to X guard, the arm drag and a quick triangle when they post.
Getting from closed guard to a front headlock: overhook, post, get your head above theirs and transfer the legs outside, then finish a high-elbow guillotine.