You fill it with water. Have you ever done a dry bong? That shit is HARSH, lol.
I know form first hand experience (I took 1 hit before we moved to a different place to do some real smoking, and we were all too stoned to put some water back into the bong when we got there, needless to say, we put some in after I did that).
And no, it's not really tedious. It's pretty easy to pack the slider properly. All you need to do is make sure you pack the tobacco tight enough to hold up the weed until you need to haul it through. And that shit fucks you up. I've never actually had the lung capacity to haul it through, but I always got all the weed (and started getting some tobacco at the end).
It's a great way to get real stoned real fast if you don't have much weed. but the first one I hit, I almost passed out standing.