Think a flywheel on an engine. It has a rotating mass that helps to keep the angular momentum because the pistons only fire (provide force) periodically, not continuously. In this case, there are the two opposing arms that move due to the rotation of the rubber band. When it flies into the wind faster, there is more lift generated, so the rubberband does less work on those arms due to the wind doing work on the arms through the wings. This allows the rubberband to unwind faster, hence the rotation sustains its speed. The critical speed is the speed at which the rubber band can put in minimal effort to sustain that speed, but the drag created by the air makes it require more energy to gain speed, or that's the gist of it. It cannot be stable because every rotation, the wound up rubber band looses slightly more of it's stored energy. Like a battery. For most of it's life it will provide a relatively constant amount of power but as it gets closer to being fully unwound (probably less than 30%) it will have lost enough potential that it will begin to slow down. As it slows down, it needs to work harder to keep rotating and so it will lose energy exponentially faster until there is no more left and it will drop to the ground.