Just like the name implies, it saves your prop. LOL
On an out runner motor the drive end has 2 small bolts that stick out from the sides. What you do is install your prop on the front of the motor, and loop several rubber o rings across the prop to each of the bolts. On impact the o rings allow the prop to move, this reduces the chance of damaging the prop and lessens the force applied to the motor. A similar set up can be installed on the output or prop shaft on a regular gearbox set up.

What it does is eliminate the nuts the mechanically attach the prop to the shaft.

Should have been standard equipment on the Slo-V