I couldn't really visualize it either, even though I had used cartos before, until I had a cheapie carto tank that didn't have the locking ring at the bottom to keep the carto *in* the tank. See, it connects to the battery via the 510 connection at the bottom of the carto; the carto tank is just a shell that rides along surrounding the carto with
juice. The carto slides into the bottom ring of the tank; you prime the carto itself with about .8ml of e-juice, then, before you slide the carto all the way to the top, you fill the tank; there's a good seal between the carto and tank at the bottom, because of the o-rings, so you don't have to worry about it just running out. when the tank is about 3/4 full, you slide the carto fully upward, till it engages the top ring; put in a drip tip, screw it into a battery/mod, and you're off to the races.
The problem is that if the carto is not "locked" in place, by an extra ring at the bottom, when you go to remove the cartotank from the battery, it's entirely possible to pull the tank upward, before the 510 connector is fully un-screwed; if you're not very careful, it's very easy at that point to completely waste whatever remains in the tank, and waste it all over the battery/mod. With the locking ring, it's threaded onto the bottom ring of the tank, which keeps the carto engaged in the tank; you can't get the carto out until you remove the locking ring.
Does that make it a bit clearer? As I said, I couldn't really visualize it either -- until I wasted half a tank of juice all over one of my vv3s. I was not a happy camper, and that cheapie tank has been relegated to the catch-all in my e-juice drawer, probably to never be used again.
Andria