Sounds like an electric motor on the trailer.
That hybrid approach is especially valuable in heavy trucking, where moving heavy loads at low speeds or in stop-and-go traffic forces diesel trucks into low-gear, high-RPM operation — exactly where fuel consumption climbs and fuel efficiency drops. Electric motors, on the other hand, deliver peak torque (and, therefor, peak efficiency) from 0 RPM, making them ideally suited for the low-speed hauling and grade change scenarios where diesel struggles most. Plus, the mass of a loaded truck-and-trailer combination makes regenerative braking far more impactful than it would be in something like a Prius.
Seems cool. I was wondering why it's better than an electric cab:
Because the VE Motion system can be attached to existing trailers and put to work behind both ICE- and battery-powered semi trucks, the system itself doesn’t ask the fleet operator to make a huge commitment in terms of trucks, trailers, or even infrastructure.