It's like this one;
You fill the bottom bit with hot water, put the filter in the top, then coffee. The spirit burner boils the water, which pushes it up into the top with steam pressure. When the last bit of water in the bottom bulb starts to boil and bubble, you take the spirit lamp away. As the bottom bulb cools, it pulls coffee through the filter. You're left with fresh coffee in the bottom and dryish grounds in the top.
I'd say it's worth it for £40, if only for the joy of watching it. Sadly, my spirit lamp's broken, so I've not used it in a while.