engine fuel primers 1969 T337D

[B2C2]

I was out to fly on saturday and my rear fuel pump wouldn't run. Normally this would be a no go, but I remembered I have fuel primer knobs on the plane and used the rear one and got the engine started. I decided not to fly due to an abundance of caution and was able to verify a loose connection back at the hangar that my mechanic is looking into. My question is this. In the POH these are mentioned exactly once, in a diagram showing the front panel and they are called out with the word optional, so must not be on every plane out there. But there is no explanation of how and when to use them. I pumped it like 10 times and the engine started. It must work because without cycling the electric fuel pump it will not start. Does anyone have these and a POH that describes how to use them?