Starting with Protosound 2, locomotives hold some of the track power in reserve in order for the cruise-control feature to work.
As the loco climbs grades or rounds curves, it starts to slow down. This is sensed by the onboard electronics, which increases power to the motors from that reserve in order to maintain the original speed. The opposite occurs under conditions that would cause the locomotive to speed up on the same track voltage (going downhill, train breaks in two, etc)
---PCJ