Engine Primer??

[Dirt_McGert]

I just bought a great condition 2003 GZ250 with 740 miles on it, and when the engine is cold, it has trouble starting, even with the choke on. So, just today I was looking to see if there was any primer that would help the motor crank. Well, on the left side where the fuel tank select switch is, there is "On" (main tank) then turned downwards it says "Pri" and to the right it has "Res." (reserve.) I moved the switch down a couple times, then pushed the electric start, and it started right up, cold engine.

Is this the engine primer? If so, how do I properly use it?

Thanks.