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So basically, the countdown during the Metal Sonic race is handled by linedef executors. This LE is always triggered on level load (although technically its trigger linedef is a once trigger, at least one player will always spawn on it, thus triggering it.)
After A minute and a half, a linedef executor is ran that sets the floor height of an intangible, invisible, space countdown FOF to -2048, covering the entire map. This FOF has a tag of 65535. Now what's special about this specific tag is that it causes the linedef special to affect every sector in a map, so you can sorta see how the space countdown FOF covers the entire map. Other maps like Arid Canyon act 2 use this tag to set a global colormap.
I hope this explains how the countdown works in Metal Sonic races.