A torque impulse delivering power tool having an automatic power shut-off (28) and comprises a drive unit (15) including a rotor (16), a power supply (24) connected to the drive unit (15), a power controller (23) included in the power supply (24), an output shaft (20), and a hydraulic impulse clutch (19) coupling intermittently the drive unit (15) to the output shaft (20), which impulse clutch (19) comprises a drive member (18) drivingly connected to the drive unit rotor (16). A retardation responsive trip device (28) includes an inertia influenced activation member (29) which is mounted on a pivot pin (30) on the drive member (18) for movement about an axis substantially parallel to the rotation axis of the drive member (18) and which is arranged to activate a latch (40-44) by which the power controller (23) is shiftable from an open position to a closed position as a certain retardation magnitude is reached in the drive member (18).