I know what you mean about “shaking each other like a terrier shakes a rat”! It sometimes looks quite alarming, but they usually seem to emerge unharmed. Luckily that seems to happen less often than the more usual biffing and rolling up. The shaking seems to happen when there is no obvious more dominant male. There seem to be a few ‘levels’. The first is one being biffed just rolls up, then sometimes one will resist but not roll up and probably be pushed along sideways, sort of half lying on his side (that always looks as if it would be quite easy to injure a leg or foot), and third, the one being attacked fights back – which can lead to the shaking like a rat.
I found that there was a fairly constant ‘pecking order’ amongst the males. The dominant male will biff all others, the second one will be biffed by the dominant one but will biff all the others, and so on. The least dominant ones will sometimes not visit the feeding area if they realise another male is present. They could sometimes be seen approaching and then do a U turn. But whilst the ‘pecking order’ held, no shaking like a rat occurred.
But yes, they are endlessly fascinating.