Every ball that goes into the center gate results in one spin of the slot machine, but there is a limit on the number of spins at one time because of the possibility of balls passing through the center gate while a spin is still in progress.
Every ball that goes into the start-up gate results in one spin of the slot machine, and the maximum amount of "credit" at any given time is 4 spins. This spin credit system is required because it is often the case that a ball will go into the center gate while a spin of the slot machine is still in progress.