Imagine you have a tulip garden, and your inbound liquidity is like the empty space in your garden where friends can plant tulip bulbs for you. If your garden has room (inbound liquidity), people can come by and plant more tulips (send you Bitcoin). However, as more tulips are planted (you receive more Bitcoin), your garden gets fuller, leaving less space for others to add more tulips until you decide to dig some up and give them to others (send Bitcoin out), freeing up room again.