Complex sentence: Riders are paced in the early laps by (and required to stay behind) a derny motorcycle, which slowly increases the speed of the race from 25km/h to about 50km/h and leaves the track with about 600-700 meters remaining.
Split into simpler sentences: Riders are paced in the early laps by, and are required to stay behind, a derny motorcycle which slowly increases the speed of the race from 25km/h to about 50km/h. It then leaves the track with about 600-700 meters remaining.