How does VSYNC work?

I under

VSYNC 1

is basically 60 frames per second and

VSYNC 2

is 30, 3 is 20, 4 is 15, etc. But how does it work? Is it just a wait command, but synced to how many frames the screen has updated? P.S. if you're curious how well it's going, GFILL is almost done(just need to add something to ignore caps), and I'm going to work on print, locate, and xscreen next.