Moving Gears / Collision Detection / Angle Dependant Mate

[LostRoss]

Just learning SolidWorks and am having some trouble with getting a moving gear to engage/disengage. As you can see from the image the first gear has a grove which moves the arm which switches the drive between the two smallest gears. How do I get them to engage/disengage correctly? I can use the collision detection while rotating but that is a bit slow and something like an angle dependant mate might work better.

Any ideas on how to acheve this?