The 510i electronic cigarette cartridge suspends the e-liquid in a heat proof fibre and the atomiser (vaporiser) element creates the vapour that appears as smoke by inserting the element at one end of the fibre. Usually the vapour is then sucked through the fibre to the end of the cigarette, this can result in the case of over filled cartridges, a small amount of liquid entering the mouth.

Not so with the 510i. The cartridge liquid container is a closed chamber and the vapour no longer passes through the cartridge fibre. It now passes down both sides of the cartridge from the vaporiser (atomiser) directly to the end of the cartridge without passing through any liquid. If you use e-liquid you will realise this means the cartridges may be filled with more liquid than usual, without this risk of liquid in the mouth. The two airways can be seen in the picture below.

Zigs Electronic Cigarette Zero Nicotine Tobacco, Vanilla & Menthol Cartridge refills provide the water, and propylene glycol allowing the electronic cigarette to create a very fine vapour that looks like smoke but without the nicotine.