Knee on belly is a top position where one knee rests on the opponent's stomach or lower rib cage, a posted foot provides balance, and your hips stay upright and mobile. It scores two points in IBJJF competition and creates intense pressure that forces the bottom player to move — often into a worse position.
From knee on belly you can advance to mount, switch to side control, attack the near arm, or take the back if they turn away. Because you are tall and mobile rather than lying flat, you can react quickly to their escape attempts.
It is a position that rewards grapplers comfortable with weight distribution and balance, and is particularly effective against bigger opponents you cannot flatten easily.