This is very good advice. Moving the mast position can transform a board, particularly it's capacity to get onto the plane, so it's well worth exploring. A useful way to visualise it is to put the board on a raised flat surface like a table and then observed where the mast sits in relation to the nose rocker. Too far forward and you'll be ploughing making it harder to pop onto the plane. Too far back and in my experience the board can get corky and stiff on the wave, but this might not be so important for FSW. Have a play and see where you end up. Cheers Jens
I'd say what shifting the mast foot back does is actually a bit different from how you have described it, and not related to rocker line. .
We are talking wave boards here but shifting the mast foot back does bring the rig connection to the board nearer to the fin or fin area. Like with a slalom board, this should get you more lift from the fin. That's because the point of leverage is brought more over the fin torque, and it's a livlier setting for early planing.
More importantly, this is about sailing stance, and shifting the mast foot back makes the rig more upright and that helps you sail off the front foot more. If instead you use the mast foot set forwards that tends to add mast rake and you get more trapped under the rig which in turn makes you over load the tail via your back foot.
People who put the mast too far forwards tend to sail in old school blasting stance and they load the tail too much which is what causes the spinout mentioned.
But I agree with the advice to experiment. On a wave board I always recommend to use the rear half of the mast track and to set the front footstraps as far forwards as possible. You then set your back strap according to your leg stride.