When you just frikkin' do it! Or when you get after it. To do something with gusto at 110%. You can nail just about any activity.
Bob: Dude I'm hungry; I could totally go for some pizza.
Carl: Nail it man! Just friggin' nail it!
Bob: I just nailed that beer... get me another one
Bob: <At the craps table just before the throw or after a win> NAIL IT!!!!! NAIL IT!!!!!