It's basically the whole bike "going into one", very violently shaking side to side, lock to lock, front to back with the rider's legs and arms slapping the tank with feet off pegs sometimes, they are truly terrifying, often go on for long enough for you to think "this is it" and start without any notice or warning, usually at higher speeds where the bike is skimming the tarmac and in the worst scenario the rider can be spat off the bike, closest I came was dragged over the front of the bike looking at the front number plate then it sorted itself out and I wobbled on .. smell it ? I was sat in it .. Not much you can do with a proper slapper apart from hang on haha.