Isn't the proper reason that pulling the downhaul on hard you can actually stretch the luff longer than the rules allow, but having the downhaul clew 6 inches above the foot you can pull on hard but the foot doesn't necessarily lower and therefore the sail remains within class rules.
Partly.
There is a black band rule. Essentially it's a function of measured luff length, sail area and foot length which will ensure you are not ALLOWED to pull more downhaul on than would streach the mainsail bigger and 15sqm (including the mast). Masts will be marked with a black band which will show the max DH that can be applied. This is the same as the A Class.
Please go to the F16 website and download the measurement spreadsheet:
http://www.formula16.org/images/stories/measurement/f16measurementform.xlsThe exact formula is defined there.
