I can trackstand, sometimes even when facing downhill by using my brakes to push me back. That said, I find it more of a neat trick than a skill worth the time it takes to learn (I dunno about other people, but it took me several months to perfect). In most cases there's no real advantage to standing still as opposed to rolling at 1mph, and if the light cycle is long enough for it to matter, you're probably better off unclipping and getting the rest anyways.