A dual-action airbrush puts two controls on one trigger: press down for air, pull back for paint. Air first, then paint, then release in reverse — that little ritual prevents spatter at the start and end of every stroke.
The pull-back axis meters paint volume continuously, which is what makes fine-line work, mottling, and soft fades possible. Single-action brushes (fixed paint flow) still exist for base-coating, but virtually every serious recommendation is dual-action.
See it in context in our airbrushes guides, or explore the tools hands-on in The Bench.