End Of Stroke. after the primary contact gets opened or closed on some leaf switches, there is a secondary switch that will be opened to do various things or another switch entirely. i believe the primary use is to kill or reduce power to the coil, although i'm new to em's as well. on a step unit, i think it stops the coil pulse after the proper rotation of the wheels.
take a look at the flipper mech's switch, that is a common EOS switch. it could easily just be that it is not aligned correctly or the contacts need buffing or are worn out if it is not working correctly. the switch diagram will tell you the proper positioning itself.