Basically it is uses cartomizers which are metal tubes with filler material and the coil built in. They were some of the original e-cig designs and you would just saturate the filler material with e-liquid and vape away. The way the cartotanks work is that the tank has two cartomizer-sized holes, one in the top and one in the bottom. You insert the cartomizer into the bottom and push it until it's about 3/4ths of the way up, then you fill the tank so that the cartomizer is surrounded with e-liquid. Then extend the cartomizer all the way up so that it goes through the hole at the top of the tank. The last step is to prime the cartomizer by saturating the filler material and giving it a few primer puffs. The cartomizer has holes punched into it so that the e-liquid can flow in to re-saturate the filler material as you vape away.