Sneaking up behind someone, estimating where their bra strap would be, pulling it and letting go, causing a snapping sound. This causes the person being brasnapped a small amount of pain, surprise, and traumatization (depending on who did it).
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.