Yas is shouted to express joy over anything. It is often drawn out (i.e. "Yaaaaaaas!") and said in a high voice. It was conceived, possibly by Nathan, as a variant on one of Amol's high-pitched screams during games like Smash.
A variant, "Naaaaas!", is used to express sadness. It is often said in a very low voice to distinguish it from Yas.
This meme inspired the League of Legends champion Yasuo.