Sorry to confuse you, though one may already be "conditioned" lol by rules n regulations...
The example being, in Formula One racing - they convert wasted energy, some guy once said "every action has an equal and opposite reaction" F1 plays that hand twice..
In understanding what happens with foil shapes on Supersonic mach 3+ military aircraft that fall like bricks out of the sky (to say the least..) at low speeds - what looks like a speedy fin in effect may as well be a brick - reverse cavitaion means a shape change on the fin to begin with but absorbs the wasted energy - not converting it to forward motion... then obviously every other drag area board / wing etc
In designing foils or 'shapes' to reform flow at speed in boiling water or supersonic speed, you can understand that shape changes are necessary thus the transformer 'brick' style fins (if you like to call them that) attain fluid dynamics..
I think the F1 example is easier way of explaining it - though not litterally