The liquid must be thinned more in these because the air and liquid are mixed inside the body of the airbrush. Internal mixing produces a finer spray. Slowly pulling back on the finger button will gradually increase the amount of color sprayed. Thus, you are able to control such things as the width of the line of area of tone, the intensity of the color or the gradation of value, all while you continue to spray. Obviously, that type of versatility isn't necessary for every task, but for artwork, it's great.
Liquid is fed into the air stream either by siphon action or gravity.
Siphon-feed airbrushes work just like sucking a milkshake up through a straw, and it takes one or two seconds to get it working. Gravity-feed airbrushes are quicker because the color cup is located on top, and the fluid flows directly down into the airstream. This is a nice advantage when making a lot of color changes. We actually stretched the truth when we said that all double action airbrushes are interal-mix types. The AB would have to be described as a double action, external-mix type.