Selection Brush and Quick Selection · Layer Comp

Paints the pixel selection, like Photoshop's Quick Mask.

Left mouse paints. Ctrl or Alt + left mouse erases.
[ / ] change the size, Shift+[ / ] the hardness. Enter or Esc finishes.
Strokes are previewed as you paint and written on release, so the brush stays smooth on a 4K frame. Overlapping dabs don't build up.

Brush roughly over something and the selection grows out to its edges.

The first stroke starts a new selection and later ones add to it. Shift adds from the first stroke; Ctrl or Alt subtracts, and that grows too, so a dab can take a whole part away.
[ / ] change the size. Enter or Esc finishes.
The selection spreads from the pixels under the stroke to pixels it can reach cheaply. Crossing a colour edge costs a lot, and so does landing on a colour the stroke didn't touch. What counts as an edge is measured from the noise under your stroke, so a textured subject grows as freely as a flat one.
It learns within a session. After one Alt-stroke over a background a stroke leaked into, the next strokes stop at its edge.
Live preview: a coarse result updates about ten times a second while you paint (red adds, dark subtracts). The full result replaces it when you release.
Each stroke reaches at most eight brush radii past its own edge, so a stroke on a big plain background doesn't take the whole frame. Paint again to go farther.
Auto-Enhance (the toggle beside the tool) snaps each stroke's edge to the image, as Refine Edge does.
Speed on a 4K frame: about half a second for an ordinary stroke.