Flow is a shoe built on Vibram's well-known Sphike sole. This is a sole specially designed for how the foot rolls from heel to toe - this is to reduce muscle fatigue and increase comfort during either normal walking or running. You can see this by looking at the sole at the arch below and is called the meta rocker.
Furthermore, the sole is designed so that the response should be medium to firm, depending on weight - so that the sole does not get flat. This also makes the shoe more stable.