Abstract: A method and a power tool system for screw joint tightening by means of a pneumatic torque impulse power tool (10) controlled by a stationary programmable control unit (22) and via a torque magnitude and torque growth calculation based on signals delivered by an angle sensing device (16) on the impulse unit (13) of the power tool (10), wherein motive pressure air is supplied to the power tool via a flow regulating valve (26) which is successively adjustable between zero and a full power flow. The flow regulating valve (26) is controlled by the control unit (22) to deliver a reduced power air flow to the power tool (10) before and during the very first delivered impulse, then delivering a full power flow until a certain torque magnitude or a certain percentage of the target torque level is reached, whereafter the air supply flow is again reduced until the target torque level is reached, and when the target torque level is reached the air flow is shut off.