the key observation is that the pin state depends on the frequency of the signal coming in. for DC, it is normally low. for frequencies *higher* than about 2.6MHz, it also stays low. but for a particular window of frequencies (1.4 to 2.6MHz) the pin goes *high*.
why is this useful? it turns out that these drives have a 1.75MHz signal recorded onto specific tracks, letting the MCU (which controls the stepper motor) know if it has gone outside of the data area.
so basically, you step around, and if you see this pin go high (and stay high) then you know you're out of bounds.