Crunch.

I have hyper-extensive elbows, fingers and knees (probably others but I haven't really checked), and I can thus bend my fingers back to just over 90 degrees, when I lock my knees and elbows out they actually bend backwards about ten degrees (I can't lick my elbows - bugger) and I can crack my arm slightly (but noticeably) out of its socket too, just by using the muscles in my shoulder.

The most common means by which I demonstrate the latter of the three is to basically tell someone* "put your hand on my shoulder" - I then crack the arm out of its socket and release the muscles, letting it slide back into place. To see the look on people's faces when I explain what they are feeling is priceless. I don't think it is fully dislocated although judging by the sensation I get whilst doing it, it probably would snap fully out if I pushed it hard enough.

Weirdly, I can operate either arm normally whilst it is "dislocated".

*Obviously not just random people I see on the street.
( Awk Industries is actually a helic- NO, STAY BACK SHARK!, Sun 21 Nov 2010, 23:10, Reply)