Corel Painter computed brushes allow media to build up or "pool" when you move a brush slowly. Pooling creates very realistic strokes, especially with airbrushes. You can also cause media to pool by simply touching and pausing with the selected brush.
With an airbrush, paint pools when you pause in a stroke for 1 second (left), 4 seconds (middle), and 7 seconds (right).
When the Continuous Time Deposition check box is disabled, you must move the brush (even just slightly) to cause media to be deposited.