When dancing with a girl at a frat/bar you place your hand down the front of her jeans and slide in Spiderman fashion (middle and ring finger inserted with other three fingers out to the sides a la Tobey Maguire). Singing the Spiderman theme song adds to the ambiance.
When you are banging a chick from behind, you cum in your hand and spit on her anus, thus making her think you have ejaculated on her, when she turns around you throw that shit at her in a spiderman-esque fashion.