You're referring to the pace bike in the Keirin event in track cycling.
Keirin takes place over 8 laps (2,000 metres). The pace bike starts off at 25 km/h and gradually builds up to 50 km/h over the course of the first 5.5 laps, after which it drops out. In those first 5.5 laps, all the riders must stay behind the pace bike and within 10 metres of the pack.
It's a way of effectively starting a race from a fixed, fast starting point leading to a sprint over 2.5 laps, rather than a sprint from a standing start.
Keirin takes place over 8 laps (2,000 metres). The pace bike starts off at 25 km/h and gradually builds up to 50 km/h over the course of the first 5.5 laps, after which it drops out. In those first 5.5 laps, all the riders must stay behind the pace bike and within 10 metres of the pack.
It's a way of effectively starting a race from a fixed, fast starting point leading to a sprint over 2.5 laps, rather than a sprint from a standing start.