6. The timeline The frame the Roto Brush is applied to is called the Reference Frame and as soon as the brush is applied After Effects creates a span of frames either side of this under the timeline of about two seconds. The selection within this span is based on the referenced frame both backwards and forwards in time.
7. Evaluate the selection Press the spacebar: After Effects will calculate the selection for the right half of the span. The selection is based on the reference frame and is propagated forwards. For backwards, move the playback head left of the frame, or to the beginning and press spacebar to render automatically.
8. Fine-tune the selection Regions where the edge does not match the object can be edited frame by frame using the Roto Brush to add or subtract as in Step 5. To resize the brush, hold down the Cmd key and drag on the screen. As you work your way through the frames the span will automatically get longer.