The separate circuit shown at the bottom is the charge state indicator in the motor section of the screwdriver.

The battery is charged through Q4 and the schottky diode in the charge indicator. When it reaches full charge, IC1 sets its OC output high, turning off Q1 and Q2, and hence Q4.

While charging, Q3 is turned on, pulling the gate of Q5 close to ground so turning it off, hence preventing the motor from running.

The motor is switched by SW1. However, in the case of the motor stalling or taking excessive current, the voltage on the drain of Q5 will rise, causing the voltage on the CS pin of IC1 to trip its over-current protection. This results in the OD pin going low and turning off the MOSFET Q5.