I see the powerline a little differently. For me the line extends from the inside of the pivot foot to the catcher. The toe of the stride foot lands on or near this line. The arm tracks on it's own line. Looking straight on or from behind at someone pitching in this manner shows the arm and feet tracking on two parallel lines.
Video is very helpful if it's possible--either from straight on or from behind.
Video is very helpful if it's possible--either from straight on or from behind.