In Mach 3 you have 2 choices- Exact Stop and Constant Velocity. In Constant Velocity mode the stepper runs full speed up to the stop point, and if you are running fast rapids and have a lot of carriage inertia, the carriage can overpower the stepper and go too far. In Exact stop mode, the stepper speed begins to ramp down before reaching the end of the travel and reduces the inertia of the carriage eliminating the possible over-run.