Another fella also mentioned it on a FB group a couple of days ago for his Placid.

I guess it makes sense for Kai surfing bloody big waves. For us normal folk, not so much. Would likely feel too 'locked in and stiff'.

Good to hear your achilles is getting there mate!
It's all part of the fun

That setup may work better on bigger steeper waves.
I have a 8'6" Hypernut that I tried all kinds of different fin variations (including the one on your board) and landed on a twinfin (using L thruster side fins {Height 127 mm, Base 120mm}) using the rear boxes. I was about to sell the board until I landed on this fin set up. The channel bottom of the hypernut gives extra hold.

I first heard about using the "power fin", aka large fin, in the back by Richard Schmidt.
surfsplendorpodcast.com/085-richard-schmidt/
I saw the Kay Lenny video where he said he liked that setup too:
I have one board that has future boxes where the front and rear box depth is the same (requires spacers to use new futures quad rear fins). I have a 7'6" quad egg that always felt squirrelly with the normal quad setup with the big fins up front. Then one day I tried a setup like Richard Schmidt with the power fin in the back, the board felt perfect. This board has the deep futures boxes in the front/rear and the front/rear quad fin set is for deep boxes, so I can interchange.
Easy to switch if you have FCS boxes.

At 226 for the mfc Kai and 250 for the futures jj it has changed surfing forever by making fins twice as expensive

Still I have to wonder if it might not be kind of good in a sup, any volunteers? I like how they reversed the futures base size so they fit the standard box setup.

In the video, Kai said, "The problem with quads, is they tend to make the board ride a lot flatter on the water"
Does anyone understand what he means? Flatter rail to rail?
Or flatter nose to tail?

Don't forget area gives lift ,I also have noticed my prone surfing mates use bigger quad fin s than I would use
The tail area is a lot smaller on a surfboard as Kia's sups are ,they are almost surfboards and he surfs them well .

if you are using wider Tail Sup and bigger sups you will not need as big a fin as the tail wide gives you life
and with bigger fins and wide tail you will get airborne on big choppy faces ,Colas uses wide tail boards and actually grind his rear fins
to help keep the tail down in the water ,I also prefer smaller rears to keep it down ,with the wider tails ,

sfilnux,fins give lift so that what he means by it flattening out the board ,tail comes up nose goes down ,
That why windsurfers have so many fins ,as to trim the board in different wind strengths ,

Interesting comments above, I always found a bigger board needed bigger fins? A board with a thinner or more narrow tail needed less fins as it could help itself bigger? in my old jimmy Lewis world wide 7'9, I tried running small thruster fins and it couldn't hold. Big sides and smaller back centre gave hold and control and turning ability

I actually had to use my wider tailed quad (c drive small) in big waves after I broke my step up. The rails gripped well due to their thin stepped design and it was fast down the line, the problem is can't get the tail railed in for good carving turns due to too much lift, this is the benefit that I believe Kai is talking about the ability to rail the board and do performance turns even at high speed. This is what the windsurf quad (twinzer) was originally known for being able to go quickly rail to rail without the big center fin. But I doubt any fin can overcome too much tail volume and width. I also prefer thruster with smaller rear or small quad rears for sup, big difference for me.

Hey everyone, I'm loving big rear quads on my 9'2 Sunova Wedge. Quiver killer