Pushing the string down behind the nut will again increase the pitch as the string is tightening, the only sort of way to get a dive without a whammy is to either detune once the string is picked or neck bend (good fun but puts stress on the neck and runs the risk of warping and snapping, particularly with bolt-ons)aha!
RIGHT!
there are several ways of doing the dive
one if you have you cant have a whammy bar
you can either
down tune it to the right pitch then really quickly go back up into perfect tuning again :S hard way
or!
go up from the nut before the bit where u wind the string and find the perfect spot and if you just put pressure downwards on the string it will create the dive
lovely