I think this is what you were thinking of jb .. fwiw flywheels store and release momentum and KE i.e. they have inertia which means once motion is given to it that motion is preserved according to Newton's First Law .. Newton's First Law, an object will not change its motion unless a force acts on it.
Beneath, B. makes it clear that while flywheels have their uses it is not they which makes his wheels self-moving - it is the constantly recharging imbalance factor of metaphorically speaking, one side being heavy and full and the other light and empty, so to speak, imo - which creates and exercises the "excess weight" and "preponderance" he describes ..
John Collins AP pg 348 .. re. Bessler rebutting Wagner ..
It also seems that Wagner is at the moment at odds with Borlach in Dresden - (he's the chap, remember, who wrote that defamatory tract with the aim of ridiculing my invention - something I still cannot forget.) For Wagner sings the praises of weights and springs, and the sort of fly-wheels to be found in saw-mills. (However, in Borlach's writings a contrary point of view is expressed.) But, fly-wheels are not to be sniffed at! Though anyone who sets about the task of bringing a Mobile to glorious completion with such devices, is not on the right track at all. For external wheels, weights etc. - all of this sort of stuff is not the real thing. The wheel's own inner force must come into being without external momentum being applied by such devices. It must, simply put, just revolve, without being wound-up, through the principle of "excess weight", as I describe in Part I. Even according to the ideas my enemies express in their writings, my Wheel is the true device, and is indeed, per se, a genuine Perpetuum Mobile. None better will ever be found upon this earth, for without the principle that I alone possess, there can be no real perpetual motion. Whoever seeks another method is deceiving himself, for my device does not need winding; it runs according to "preponderance", and turns everything else along with it; so long as its material shall endure, it will revolve of its own accord. On one side it is heavy and full; on the other empty and light, just as it should be. That which hitherto has been impossible, was vouchsafed to me to discover.