A prop saver is a very simple device that bolts on the motor shaft in place of the prop. The shaft is then cut short so it barely enters the hub of the prop. You install the prop on top of the prop saver, then stretch a small rubber band or O-ring over the propeller to hold it against the prop saver. When you hit the ground the theory is that the rubber will stretch a bit and allow the prop to flex instead of breaking. The key is getting the right about of stretch. Too little, and the prop will come off in flight. Too much, and it's just as bad as bolting the prop directly on the prop shaft.

That said, prop savers on anything but small park fliers (Speed 300 and smaller) won't work very well. The bigger planes develop more thrust, more power, and need a stronger rubber band to hold the props on. Problem is, the rubber band is too strong to let the prop flex on hard landings.