How the duaphragm tap works:
 The air/fuel mixture is sucked into the combustion chamber, the small tube which you will usualy find on the inlet connection the barrel and the carburetta on pot No.1, the other end runs to the pertrol (gas) tap. also sucks air from the tube which, inturn, pulls the rubber diaphragm in the tap, opening the tap and allowing fuel into the carb(s)

If you want to know what the "prime" setting is for, and when to use the prime setting:
 Use prime when the machine has not ben run for a few days/weeks where fuel has evaported from the carbs.
Use Prime if you have to drain fuel from the float bowls. It lets the carb(s) fill faster lesseinng strain on the starter motor and means your battery will cope easier.
 If you run your machine when prime: You will probably use more fual than normal, worse case, are more chance of fuel leaks, especialy when you park your machine
 

On this type of pet cock the options are: "Reserve" (the shorter of the tubes which goes into the tank), "On" (the longer of the tubes which goes into the tank), & "Prime"(which opens the diaphragm and lets the pet cock work as a gravity fed tap). On some machines there is no prime which is replaced with an "Off" switch, stopping any fuel from leaving the tank.  When "On" or "Reserve" the diaphragm is supposed to stop the fuel leaving the tank

On the back of the tap you will see four small crosshead screws, by removing these lets you check the diaphragm.

It's a good idea to hold the casing in position while doing this as there is usualy a spring underneath the case which can get lost too easily.

The Diaphagm it's self comprises of two rubber flaps over a central piller with a o seal and a plate, if there any splits in the rubbers, or any signs of perishing, replace the diaphragm, which are not too expensive.

On the left you can see the filter which is usualy inide the petrol tank, the smaller of the brass pipe connections is the diaphragm tube fits.

A stripped Diaphragm tap