That ability makes no sense. No damage from any Rocks on switching in? Why is switching in so special? Why not just give it Rock Absorb, and finish the job off. IMO, being able to switch into TTar, Gyara, Hippo, etc, without taking one bit of damage if you predict right, is shattered beyond belief.
Just make it no indirect damage on entry to battle, ie, SR, TS, Spikes. Simple, easy, isn't broken, and makes sense.