basically its a tube that attaches to the value and sits inside your co2 bottle...as the bottle lies on its side, usually as a slight angle, the tube is bent so that it doesnt dip into the liquid co2 which will naturally sit at the bottom of the bottle
they are not flawless tho, if you tip the bottle they will work in reverse and draw liquid, but in general they are a good 'first measure' to stop you getting your cocker full of liquid....ive ran a cocker for years with an anti-syphon bottle and an in-line reg with no probs.