To grab someone's side with your hands via swiftly bring the hands together, making it seem like your flanges are shark jaws "biting" the victim. I do this to get all my friends.
A second version of this is called the shockbite, whereupon you shock the victim in the side with a hand held in the shocker position.
1) *people having a good time*
*sharkbiter walks in*
*Sharkbiter grabs side of nearest girl*
Everyone else: WTF mate?
2) *everyone having a good time*
Shockbiter: "Hey Colleen."