thats the murder beam glitch. me and jeff royally fucked with the game using that same glitch. I even found out how to get to the very begining of the game with upgrades i shouldnt have at that point using that glitch. I could never figure out how but when you use the murder beam glitch like during a boss fight like he did, it perpetually hits the boss. thats why mother brains head wouldnt stop flashing. When ever i tried using the murder beam glitch on mother brain I just end up locking up the game...