Moving the Z-axis stepper motors below Z 0.0
To allow negative axis values, usually, the way is to use G1 S1 to disable boundary checks and G1 S0 to enable them.

Another way is to force homing Z, which means moving till an endstop triggers and then moving back up the specified motion via G28 Z

Another alternative would be to send G92 Z50 to set the Z-hight to 50, then G1 Z-50 F200 to move that much down, rinse and repeat.