[Zmetta]

The valve is based on the Venturi effect. The fluid flow through a constriction lowers the pressure at the constriction point. An inlet at the low pressure point will allow addition fluid at atmospheric pressure to enter through the inlet in the constriction thus adding to the total fluid through the outlet. Pretty clever use of fluid dynamics!

[tlrobinson]

Avalanche airbags use the same principle to inflate larger than the volume of compressed air in their canisters.

[notdrunkatall]

This is what I was looking for, thanks.