Adobe photoshop 6, is there are tool to cut out anything outside a the lines edges

I was watching an art video from the Behemoth and I noticed as he is coloring in the moosecoon he just runs a wide brush stroke over the area. The lines often go outside the black outline for the unholy beast and it looks like and makes a mess of it. Right after he lets go of the tool the sections of the brush outside the black outline are removed. So my question is how is he removing it or is it just him erasing the lines and I don't see it because of the play back speed.

for reference the video is Youtube, screen capture