Hydraulic Actuator Null
I believe you are referring to hydraulic null and mechanical null for a hydraulic boost actuator in helicopter flight controls.
Mechanical null is simply the half stroke displacement of the control valve spool between stops.
Hydraulic null is the control valve spool position where the spool is positioned equally over the ports feeding flow to extend and retract the actuator. At hydraulic null it takes the sale spool displacement to initiate extend or retract motion.
CTR