When a chick is grinding, rubbing, and thrusting her beaver (pussy) against her lesbian (bi-curious?)partner's beaver. Term "smashing beavers" is used when this activity becomes quite aggresive and smashing occurs between clitori in order to facilitate maximum stimulation.
The nonverbal signal for this is to make a "Spock" gesture with both hands, and smash the crux of the V's together.
Tiffany and Veronica started out slowly rubbing their vaginas together on my couch, but as they both neared climax, they got so heated up that they began smashing beavers. I could hear it from the kitchen!
I make the smashing beavers sign with my hands everytime I see lesbians to denote what their favorite activity is to my friends.