Sometimes a picture is worth a thousand words. Here's the earliest prop saver thread I know about, from 2001.
http://www.rcgroups.com/forums/showt...threadid=20421

All prop savers, whether homemade or commercial, are based on the principles shown in the thread. Prop savers are not to be used with motor producing more than about 100 watts. O-rings are better than rubber bands for holding on the prop.

- Jeff