In all versions of the game, when you enter a certain planet for the first time in that session of the game running, it plays a rendered cutscene introducing the region. However, since Dreamcast loads are so slow, it is only really relevant here, since loading these cutscenes takes about 3 seconds each time.

So what you do before starting a run, you enter a track on every planet once, to skip the loading of that cutscene, which will save quite some time over the course of the run.

The 100% route involves a console reset. If you run 100%, you want to route it so that the reset comes as late as possible, to have the cutscene skip active for as long as possible, since for every race you do after the reset, the game will load the intro cutscene. For more info on the 100% route, go here: http://www.speedrun.com/swe1r/guide/p9xqq