To get an immediate erection from a lovely specimen of the opposite (or same) sex (whatever floats your boat).
Damn that girl in volleyball shorts gave me an instarection before I knew what happened.
When a male prematurely ejaculates BEFORE even becoming erect.
nerd: dude that hot girl just gave me an instarection! she didnt even touch my penis and i sprayed my load before i even got it up!
shane: ... you're a lil bitch