A kneebar in BJJ works exactly like an armbar -- except you're hyperextending the knee instead of the elbow. You trap your opponent's leg, position the knee against your chest, and extend your hips to apply pressure. Simple concept. The details are what make it work.
Every leg lock has three parts: the entry, the entanglement, and the finish. Here's the finish by itself so you understand what you're building toward.
Start in reverse mount -- sitting on your opponent's stomach, facing their legs. Straddle one leg so it's between yours. Hug the leg, pull it up, and fall off to the side.
Now here's where most people mess up. The number one error is too much space between your hips and their body. If you're stretched out, you're cranking on the shin, not the knee. That's a shin bar, and it's not going to tap anyone. Keep your bottom CLOSE to them.
For the grip, lay your hand on their foot and control the heel. Why the heel? Because the knee follows the toes. Point their toes toward your chest, and the knee points toward your chest. That's the angle you need for the break.
Cross your feet, make sure the knee is up on your chest -- not down by your stomach -- and slowly extend your hips while pulling the foot. That opposing pressure is what gets the tap.
Three grip options work here: heel control with hand on the foot, figure-four grip, or a standard gable grip around the ankle. All three finish the same way -- hips forward, foot pulled back.
For the complete leg lock system including entries and transitions, check out our full guide to BJJ leg lock entries. Get the full course in Scott Sullivan's BJJ 101 System.