The idea of non-rocket propulsion. There's also a discussion going on here.
This is interesting. I actually thought about a propulsion system like in their diagrams, when I was a child, but couldn't think of a way to vary the mass of the piston on each half-stroke. I think I thought about pumping a fluid back and forth but that would be slow and would still be subject to Newton's Laws.
I can't get no satisfaction
I can't get no delta-v without reaction
I didn't see how the piece's discussion of Mach's theories of relative mass really supported anything. One does speak of things such as "rest mass", for example.
I didn't get how capacitors and piezoelectrics produce the variable-mass piston. I guess you could imagine that a charged capacitor contains more energy (and more electrons) and therefore a tiny amount more mass, and if capacitors which were charged were shifted in the opposite to the direction of travel, discharged, then shifted back over and over with piezoelectrics, it would yield some non-zero thrust.
Did they actually say that? They said the amount of thrust would be hard to measure over background noise. I bet. A piece like this should have pictures of the mechanism and some numbers about its function, since it had other details, just not that.
Capacitors are interesting things, simple, but interesting. It would be interesting to know their input and output numbers, if they have any. Oh, I could probably throw together some theoretical values for sample capacitors and piezoelectrics. Might be fun, no?
I'll try to look up displacements for piezoelectric materials and charge capacity of typical capacitors and mass of electrons and also energy to mass equivalences to get a ballpark idea.