and the pdm back pressure is sensed by the pressure control valve 48 the pistons apply the downward force until sensed downhole pressure reaches 650 psi for example which represents a desired working pressure the pressure control valve then switches the anti stall tool to the passive mode when sensed pressure reaches the desired drilling pressure of 650 psi for example here the pressure control valve 48 shuts off flow to the pistons and hydraulically locks the pistons in the passive wob mode the pressure control valve 48 is closed and no pressure is sent to the pistons the pistons are sealed and existing force is transferred to the drill bit motor pressure is not increased downhole pressure continues to be monitored in the passive mode via the vent valve 54 and sequence valve 52 which monitor pressure change in the coiled tubing the passive state continues until sensed back pressure reaches 800 psi for example once downhole pressure reaches the 800 psi level the anti stall tool switches to the reverse mode that is if torque in the pdm increases it causes an increase in back pressure motor stall is prevented by sensing and reacting to back pressure at a level below motor stall e g 800 psi or other pressure below that at which stall can occur when sensed pressure reaches 800 psi the.