Hi,
Mach only backs off so the switch unmakes, usually only a fraction of a mm from where the switch first operated.
The sequence of events:
1) If the Z axis is the first axis to home as declared in the homing order setting
2) The Z axis drives in its nominated direction and at its nominated speed until the Z axis home switch makes contact
3) The Z axis stops and then reverses until the switch un-makes, usually a fraction of a mm
4) The Z machine coordinate is zeroed, or set to the Home Offset value if so programmed.
5) The next axis in the home sequence is activated.
Note in step 4) if the home offset is set to 25mm say, the Z machine coordinate is set to 25mm, the axis DOES NOT back off 25mm.
I have in recent days been told that one of the popular motion controllers can, if programmed, drive to a 'back off' location. This is done by the controller
not Mach. I think it might be the UCnnn series from CNCDrive. Note it is the controller that does this not Mach and is not standard Mach behavior.
Craig