The DYAD consists of a fuselage suspended from two large motorized wheels. When the motors are turned on, the inertia and rolling resistance of the wheels is lower than the force required to swing the fuselage around the axle, so the vehicle moves. By independently varying the speed of the wheels, the vehicle steers.
Each motor is actually inside the wheels, near the center. A strong Technic framework connects the motors (and two counterweights) to the fuselage. Half cylinders cover part of the framework to allow the rest of the wheel to roll over it.