https://www.youtube.com/watch?v=rfv9auQ-_fo

What happens if I use that together? Will it work, but with less smooth shifting?

Won't work. Buy a 11spd cassette that has a lose 16t cog, and remove that one. Adjust limit screws and derailleur cable. If you swap back and forth with a true 11spd wheel, it's a PITA.

Won't work right.

Some far better options explained here:
http://darkspeedworks.com/blog.htm

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It has actually worked just fine for me for over a year on my climbing road bike.

I used a SRAM Force 22 RD with a SRAM 12-32 cassette for the last year on my climbing road bike. 10sp chain. 10sp cassette. 11 RD. 10sp shifters. Doesn't just work, works just like it should. No delay, no hesitation, no skipped cogs, etc.

Maybe I am lucky thus far.

That works like it should because there's no difference between the 2012/2013 (10 speed) RD and a Red 22 RD.

Ah. Somehow missed the Shimano part of that original comment. Apologies. Hope it works for you.

Ummmm, get an 11 speed cassette with an 11 speed spacer and it will work fine on a Shimano 10 speed freehub... won't it?