The suction starts on the rapid setting to simulate the way the baby sucks to get the milk flowing. I watch the bottles and once I see milk dropping in I push the letdown button which switches the suction to stronger but less fast, like a baby does once milk is flowing and they're sucking/swallowing. For me the letdown always happens before the 2 min are up and the pump switches by itself. Anyway, that's how I use that button...
Re: Let down buttons - I'm confused!