The cheer comes from Five Nights at Freddy's, the 2014 indie horror game created by Scott Cawthon. It plays when the player survives a night and the clock reaches 6 AM, rewarding hours of camera checking and animatronic scares with an oddly innocent celebration. Clips of that moment circulated through gameplay videos, remixes and soundboards, and the isolated yay became shorthand for surviving something stressful. It remains popular because the contrast between cute voices and a tense horror game still works in unrelated edits.