They're just two microswitches, and each has a diode connected to it. This allows power to travel in one direction only. The idea being that when something reaches the end of its travel, it hits the switch which cuts the power to it, thus protecting the mechanical bits. But it only cuts power in one direction, allowing you to reverse/retract the motor to reset or reposition it. A microswitch at the other end of the travel will cut power there too. I fitted these on my featherweight grabber bot and they worked pretty well.