If you look carefully, you can see the yeast cells partying on those rafts. Its kinda like yeast club med. They all come out to copulate, then they lay down on these rafts, catch the rays and fart and pee into your beer. When they're done, they'll turn around and eat the rafts, then sink to the bottom and take a nap until you wake them up with more food. Kinda like frat boys, except they clean up the mess.