You can get slight vibrato or a slight bend by grapping the headstock.....

It's mosty done with fixed bridge guitars....I use that technique everyonce in a while with my SG.

Everyone who reads this, hit a natural harmonic, grab the headstock with your left hand and the body of the guitar with your right hand (vice versa for southpaws) and hold it away from your body. Just simply shake the guitar and it slighty bends the neck to give a slight vibrato.