In Hold mode the vehicle should stop and for regular steering-throttle rovers, the steering will point straight ahead.
This is a good mode to Arm and Disarm the vehicle in because the transmitter inputs required to arm/disarm will not affect the steering or motors.
The various Failsafe features are often setup to switch the vehicle into this mode.
The output to the servo and motors will be the values held in the SERVOx_TRIM parameters (i.e. SERVO1_TRIM, SERVO3_TRIM)