In playing with the tapping head by hand, I have been able to
make it reverse direction but I am unsure of exactly what makes
it change direction - is it torque based or is it when the unit
receives an axial load (tap botomed out)?

The ones I have seen reverse direction when the axial
pressure on the top changes direction from downfeed
to upwards, so that you can forward and reverse by
pulling up or down on the drill press quill feed handle...
--Glenn Lyford