Load up ONI: Sword Base with at least two players, on any difficulty. To activate the Reach Racer easter egg, you need to have each player stand in specific points on the map and hit secret switches simultaneously.
The first switch can be found on the coast, southeast of the buildings holding the COMM Array, on a pile of rocks near the frozen waterfall, left and a bit back from a small tree. It should say "Hold
to open door".
The second switch can be found past the Anti-Air cannon, on a pile of rocks with some trees overlooking the walled complex on the ice below the area. It should also display the same text.
Have bot players hit their switches simultaneously to be teleported to the Reach Racer easter egg.
For those that prefer a video guide: