What is the lever on the head of my floor pump for? I have a Wrench Force floor pump.  There is a lever on the head of the floor pump which can be moved into one of two positions.  What is this for?

The pump automatically selects the correct valve and there is a separate button near the pressure guage to release pressure so it is for neither of those functions.
You press the chuck onto the valve, then pull the lever up to lock the chuck to the valve.  Otherwise pressure could blow the chuck off on higher pressure tires.