This high pitch sound from 2:26 is called pitch snare or river' snare, this is a kind of spit snare. River' and Colaps also have a tag team“Rogue wave”. They participate in gbb 2021 tag team category too.
As for how the people know this song and what's gonna happen, you get into GBB by doing Wildcard videos which are selected. River' wildcard was No.1 ranked.