Ok, maybe I should explain - it's where you take all the crap gifts you were given, and bring them to a gathering (along with lots of booze), and the gifts are all put in a pile, and everyone takes it in turns to pick one until they're all gone. (Sometimes you have to have a drink every time you pick a present.) Then, everyone in turn gets to pick a present from someone else and give them one of their crap ones instead. You can't refuse to give up presents or refuse to take crap ones. You go round like that until everyone's either too blitzed to continue or until a set time has run out.
Someone else's crap may be a great prezzie for someone else!