Hi,
it fundamental to Mach, in fact all CNC machines if an axis moves then the machine co-ordinates change.
If your Z axis is moving and your Z axis machine co-ordinates are not changing the your copy of Mach is broken somehow.
G31 is coded in current work co-ordinates. So:
G31 Z -0.5 F10...... will cause the Z axis to drive at a rate of 10 in/min from its current location to -0.5 inch or until it detects a probe strike.
Craig