When someone's mounted and the top guy's raining punches down, if bottom just covers his head the fight is stopped because he's not defending himself sensibly.
But if he were to try to knee and elbow escape, he'd leave his head open for blows - now that's hardly sensible.
And if he wants to bridge him off, he has to grab an arm - but for that he has to wait for the right opportunity. And that's about timing. But in that time the fight will be stopped.
So what's the best defence to punches in the head - assuming you don't want to open youself up to even more punches int he head?