What is Underhook in BJJ?

An underhook is a fundamental grip where your arm goes under your opponent's armpit and you control their shoulder or back. "Winning the underhook battle" often determines who advances position. From half guard bottom, the underhook lets you take the back. From top, it helps pass guard. In wrestling and no-gi, underhooks are crucial for takedowns. The single underhook and double underhook positions are foundational in grappling.