I have one from Dennerle that has two plastic flaps doing exactly what BarryH said. It takes a bit of practice to learn how to start the flow. It's not that bad but if you manage to suck up sand, you'll need to clean the flaps before a re-start. Any stuck particles will prevent the flaps to fully close and the air/water can return freely.
I don't rate them, I always seemed to make a mess of the sand/soil with them when starting.
Just my experience of them.
I far prefer either a simple siphon one or a piece of hose already filled with water so the siphon action can start straight away (I hop this makes sense?)
I also play around a lot with hozelock type hoses and fittings to add in connections and control on/off actions.