figure four leglock
The wrestler stands over the opponent who is lying on the mat face up and grasps a leg of the opponent. The wrestler then does a spinning toe hold and grasps the other leg, crossing them into a "4" (hence the name) as he does so and falls to the mat, applying pressure to the opponent's crossed legs with his own.
While the hold applies pressure to the knee, it actually can be very painful to the shin of the victim. While the move is primarily a submission move, if the opponent has his shoulders on the mat, the referee can make a three count for a pinfall. If the referee is distracted, heel wrestlers may grab onto the ropes while executing the move to gain leverage and inflict more pain.
With enough strength and willpower, the wrestler on defense can flip himself (and also their opponent) over onto their belly, which is said to reverse the pressure to the one who initially had the hold locked in.
This is innovated by "Nature Boy" Buddy Rogers and popularized by Ric Flair. Flair's daughter, WWE superstar Charlotte Flair, uses a bridging variation which she calls the Figure-Eight Leglock.