There are a few well known party games where players stand in a circle and take it in turns to do something and then get another player to do the same thing (or related thing) by pointing at them or saying their name. The Stupid Ninja game is therefore, at first glance, not that different...but the addition of ninja noises as well as adjacent players activity raises it to a different level.
The starting player makes a ninja 'hai!' whilst pointing at a player elsewhere in the circle. That player must then put their hands together in a samurai sword shape whilst emitting a ninja grunt. Adjacent players must then, simultaneously make their own ninja arm swords and aim toward the new player's stomach, again with their own sounds. This player makes a second ninja 'hai!' as he redirects somewhere else in the circle. The video makes it perfectly clear. Players slow to respond are out of the game and play continues.