Didn't hear anything about this, so here it goes. When the Thwomp almost starts his attack, crouch. Then when he goes in about a 90° angle Back-flip. You will teleport over the Thwomp and then you're free to attack the bastard. Pretty easy, i think with good timing it could be used in speedruns. By the way, i never watched any Super Mario 64 speedrun so this might be an old trick or something like that. Cheers!