from what i know about that device, this is my take on it: when the bike is off, there is no vacuum applied to the 'in' side, aka the little tube that connects to a small black vacuum line that connects to cyl#4 (right front) when the bike cranks over & fires, it creates a vacuum and pulls the diaphram tighter and tighter allowing fuel to start to enter the other side of the diaphram. the fuel is held back by the large black rubber stopper on the opposite side. the purpose of the little rubber 'ball' i think is to block the flow of vacuum, or to keep maximum vacuum applied to hold back the diaphram during operation. if it were not there, i would assume that the diaphram would pulsate with each vacuum pulse from the engine and cause sporatic fuel flow. when the engine is turned off, there is no more vacuum source, so the diaphram closes off the fuel flow via the return spring, and blocking the fuel from flowing down to the carbs. i've tested mine (when i actually rode with it on the bike, have since removed 2 years ago) and it did not shut off instantly--it took like 4 or 5 seconds to completely shut off the fuel flow. if yours does not shut off after 5 seconds or so, something is wrong, either a torn diaphram and fuel is leaking into the vacuum chaimber and allowed to drain into the engine (i've read about this last year) or the return spring is weak, not shutting off the fuel, or the vacuum 'in' port is obstructed in some way. this is just a saftey precaution valve, you don't actually need it for the bike to function.