left, top, right, bottom Values relative to the coordinates of the movie clip's parent that specify a constraint rectangle for the movie clip. These parameters are optional.
The problem that I have with that is the mask stops following the mouse outside of those specified boundaries, BUT it still tracks it from within the boundary.
EX. If my mouse is outside of the startdrag area, and I move my mouse horizontally, the mask tracks back and forth below within the defined area.
where 200 and 400 are you wanted boundaries and check is your instance name. and in your second frame just put a frame there to keep updating the actions.
i have an example to download if you want.