The internal components with the fasteners and outer shell removed. 

When the injector is pulled from its housing the main piston is pulled backwards locking it into place. This creates negative pressure in the chamber on the left. The negative pressure pulls the stopper from between the two chambers allowing the carrier solution and the glucagon tablet to mix. 

When the user presses the pen against the skin of the patient the orange safety tip is forced back to releasing the locked main piston. this allows the piston to move all the way forwards injecting the solution into the patient.