Answer:
The Thompson uses a type of blow back action. The bolt is pushed back by the gases propelling the bullet forward through the barrel. The bolt stays closed until the bullet leaves the barrel due to a combination of friction, inertia and the pressure from the recoil spring. Once the bolt starts moving back it extracts the fired case, runs it into the ejector to throw it clear and compresses the spring. The spring then returns to bolt to battery. The bolt moving forward strips a fresh round from the magazine and pushes it into the chamber.