Rotobrush

[ingyaningya]

When I make a selection, and move the frames ahead, the selection keep overlapping into the background, or moving inwards into the object I'm trying to select. I only want it to move when the object makes a drastically noticeable change, and not re-calculate until said movement is made.
(If it helps, me video is just that of a man talking, and his head move a little, and it is the head I'm trying to capture)