Update! Click here (20 Brush and Painting Tool Shortcuts in Photoshop CC) to see twenty of my favorite tips related to brushes and painting in Photoshop CC in a single post.
Option + Shift + P (Mac) | Alt + Shift + P (Win) toggles the airbrush attribute on and off. When the airbrush attribute is in use, tapping a numeric key will set the Flow amount. If the airbrush attribute is not applied, then the opacity is affected. Use the Shift key to target the other field.