Using a magnet to destroy you're harddrive in the event that the FBI, police, or anyone come to take your harddrive and use it against you in court.
Bob got caught sharing music. As the police came to arrest him, he started magnetting his harddrive, and external harddrives, so there would be less evidence against him.
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.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"