Pumping the Throttle makes the accelerator pump squirt and extra few cc's of fuel into the airflow through the carb. This is to enrichen the mixture.
If theres no airflow the fuel runs down hill back through the carb and air intake filter and into the engine cowling. This all depends on the amount of pumping you do.
Just need a source of ignition and you've got a nice intake fire!!
Some a/c do not have accelerator pumps, some a/c have no primers and rely on the accelerator pump to prime. Know your a/c
Another common problem is one of the conventional primer nozzles is blocked and the cheat fix is to pump the throttle to make up for the difference