What is an underhook in BJJ?

An underhook threads your arm under your opponent's armpit and up their back, controlling their upper body from inside their arm. Underhooks are fundamental for control in wrestling and BJJ. Having an underhook usually means you can control posture, prevent takedowns, set up your own attacks, and escape bad positions. In half guard, getting an underhook is often the first step to sweeping or taking the back. Standing, underhooks help with takedowns and clinch control. The battle for underhooks is constant in grappling - whoever wins the underhook battle usually wins the exchange.