You sit astride your opponent knees up high under armpits to avoid being bucked or alternatively lying on top of your opponent grapevining their legs with your own whilst your arms act as stabilisers and your chest smothering their airways.
You sit on top of your opponent, with their knees up high under your armpits to prevent being thrown off or alternatively, you lie on top of them, wrapping their legs around yours while using your arms as stabilizers and pressing down on their airways.