If a circuit is already precharged, then it's upkeep (to offset thermodynamic losses and conversions) is rather easy by parametrically altering one or more capacitors' values, lowering and then raising, their capacitances very abruptly in the milli Farad - or more - range (serving as the upper boundary). In this example, I suggest the use of 100 milli Farads as the upper boundary and 50 milli Farads as the lower boundary. Timing is also important. If you setup the parameters of variation beforehand (as described above), and then flick the solitary switch (in this example) at just the right moment immediately prior to a sinusoidal wave turning around (falling after rising or rising after falling), then you increase your chances of a gainful result rather than a loss.
This endeavor is much like pushing a child on a play-swing at the right
moment to amplify her arc rather than diminish it (to avoid pushing her
at the wrong moment in her swing). The difference, here, is that pushing
a child – or having the child lean forward – drives an oscillator by
injecting new energy into the pendulous swing rather than parametrically
amplifying stored energy by having the child stand up and sit down
suddenly (from a squat) just before reaching the apex of her arc – or at her apex – (finishing this procedure of shortening and lengthening the radius of her arcing swing before reaching the apex of each swinging arc so as to amplify, rather than diminish, her stored momentum).