How do I do this?

[Hatterasguy]

http://www.youtube.com/watch?v=x1h5nuRxLnM&feature=related

Looks like you just nail the stock to get it to flip out, I'm so trying this later.

 

[9mmepiphany]

It's applying force to a lever and letting inertia work...there is only a friction lock to overcome

 

[danprkr]

Just with an unloaded gun please. I'd hate to read about it in the Darwin Awards.

 

[CoRoMo]

Fancy trick, but not at all useful.

A 'trick' like this though, would be quite useful.
http://www.youtube.com/watch?v=Hgdq1FBYTUE
Practice this one, I dare you.