At its most basic, the idea (as I understand) is that if we push on a mass when heavy and pull on it when light with the same force there will be net movement. The fluctuation in mass is done by fluctuating the internal energy through electricity. The transducer provides the push and the pull.
In the diagram, when the transducer expands, the mass in the front one makes it slightly lighter and the back one slightly heavier. This causes the front one to accelerate a bit more than the back one. When transducer contracts, it also corresponds to the mass fluctuations such that the back mass is lighter and front is heavier. So the back one moves a bit more in the direction of contraction than the front one. This way the object moves slightly forward.
This is how I understand it. Here is an interview of Dr. Woodward on this topic (in Predict)