Kids, ever have parents walk in on you when you're supposed to be doing homework, instead you're playing a full-screen game? Adults, ever had a roommate/spouse/friend walk in on you as your viewing a hot A2M porno? Well then, you gotta employ some ninja fingers!
Mom: Timmy, are you doing your essay?
Timmy: *Alt+Tab* Yes mom, i am!
Rick(walking into Dick's room): Hey, did u catch the game last night!?
Dick: *Alt+F4* Yeah dude, that was awesome!
They both used ninja fingers to protect themselves.