Death Warp Abuse
A Death Warp is when you fall into a death plane (an abyss, water, etc) so that you are respawned elsewhere. In speedruns, various spawn points from death warps can be used to our advantage, either to reach certain areas earlier than intended, or to reach a destination quicker than if we had skated normally. Each death warp takes away a bit of your character's health - reckless use of death warps may end your run.
Death Warps are most obviously used in Skyscraper District, but are also beneficial in Rokkaku-dai, Bottom Point, Chuo, and several other levels. More detail about these warps can be found on each level's respective page.