CG got it mostly correct. The only difference is that there is no set screw. You slide the prop adapter onto the motor shaft, slide the collar over it, put your prop on, and tighten down with the prop nut. The pressure from the prop and prop nut will tighten the collar over prop adapter, squeezing the prop adapter securely onto the shaft (there are grooves to allow the prop adapter to bend and squeeze onto the shaft). It seems a bit odd at first, but it does work well. This is also how the Parkzone Bf-109's prop goes onto the motor. I just finished up the E-Flite Cub 25e with the Power 25 and got 2 flawless flights on it today. I've never had any issues with this prop mount system.

Hope this helps.