When you make you hand in a cup shaped position and fart in it, shortly after farting in it you push it in your friends (or someone you don't like) face covering there nose and/or mouth.
It is a very good pranks and makes the victim smell your dirty farts.
This prank was origanally made from a Ukrainian prankster and it is like a gas mask when you put it on there face therefore it is name "Ukrainian Gasmask"
Slangs for this term include "The Dirty Ukrainian" and "Sour Cabbage Surprise"