i was lucky enough to be tuning my irvine .53 ( i know it is a small engine compared to what you guys use) but my hand slipped onn the needle and the prop went "BANG" the very tip of the prop (about 1/2 inch) embedded in to my right index finger (on the knuckles) and it stoped the engine.
it hurt like hell and i have a nice dent in my knuckle.
so even if you stay behind the prop STILL BE CAREFUL!!