You press down on L3 and then push towards the direction of the dude knocked down. That happen to me a lot before. If by the time you get there he has recovered but still down, if you flick the right stick then press Triangle or Square he will do a diving punch. And possibly you could rock him again or KO him (from the diving punch), but normally you just punch him and fall onto his guard.
Ah thanks man, I gotta try that when I get the chance.