Paddle length for flat and downwind

I’ve recently watched a video with the paddler instructing the lengthening of an adjustable paddle for a downwind session. I was always told that I should shorten the paddle to get the cadence up and to avoid getting ‘snagged’ on the swell. What is the science behind paddle length for different conditions?

No idea about the science but for me shorter upwind, longer downwind but not by a whole lot (208-210 for me). Try it - go to extremes. Too short upwind not enough drive, too long long upwind hard to keep cadence up for the slop to-ing and fro-ing for me. Too short downwind not able to generate enough power to get on runs, too long can't sustain the effort (old man).

Hello Everybody,
I watched this video few month ago and I made some try with a longer paddle in downwind than on flat. It's logical because your are pushed by the wind & wave, in theory you need less effort to go at same speed, you can paddle with more length with the same effort than a shorter in flat. But in practice you need sometime to push harder and quicker to get the run, and when your paddle is longer, you can't do it, or if you can, you can't for all of your downwind. I goes back to my standard length (209) in my last downwind and I can push hard on the 22km of run. It's look easy on the first 5km but it's perfect for the rest of time, and like that you have some juice for surfing through the end.
Sylvain.