It will mix the paint and thinner, but depends on the paint, definitely can't really do it with Vallejo, as all it does is create big ass bubbles in the cup.

I just mix everything in the airbrush, put in the thinner first, then the paint, use an old brush to actually mix the 2 in the AB....works everytime.

Also depends on the airbrush you are using. Seems easy to do with an Iwata, but the H&S brushes don't seem to be able to back flush that easily....or maybe it is just me....I usually don't get a good seal, and when I do try to get a decent one, I end up stabbing myself with the AB needle.