Phil and Danielle from getwindsurfing.com have published another excellent video - this one is titled "Duck to Backwinded":
As usually, Phil makes this seem easy. For me, learning the light wind version (which you should learn first) was definitely not easy. However, the version I learned is a bit different: the mast gets pushed further down to the water, and the sail is supposed to float for a little while before coming back up. In most of the ducks that Phil does in the video, there is no float; the time that your hands are not on the boom are minimal. That's how the high wind version is done for tricks like the Switch Kono or the Burner most of the time, judging from freestyle videos.
Phil emphasizes to pull the clew to the back to stay in control. That makes perfect sense. I have definitely had the sail load up and/or flipping around the front, as he shows in the video, many times when I learned. The other way (push mast towards water; backwind a bit so the wind pushes the mast to the tail; spike the clew towards the water) is definitely possible. But I think it's a lot harder for several reasons: (a) you need more control while luffing from the clew and then backwinding with the mast near the water; (b) the timing needs to be very accurate; and (c) the clew throw has to be done with just the right force - too little, and the mast hits the water; too much, and the clew hits the water, or the sail loads up. In very light wind (less than 5 mph), the "floaty" version becomes close to impossible.
The only thing I would add to the suggestions in the movie is that practicing some ABK-style sail chi on shore will definitely help get this move faster. The mast must be sliced into the wind neutrally from the clew; that's not exactly trivial. However, practicing clew-first luffing and other sail chi exercises is a great way to learn this skill. This is time well spent - the light wind duck tuck is one of the coolest ways to turn around, and it scores highly at freestyle competitions like the East Coast Windsurfing Festival. The ECWF Long Island is coming up in less than two months - so start practicing!