Anyone have feedback on the logistics of the Mosaic “even more space at check-in” benefit?

I am sure some routes tend to have more availability than others, just curious if this is like a mad scramble to checkin online first to secure these seats.

I have a couple transatlantic flights booked for 2024 (which a even more space seat is about $400/rt and I feel this is about the only value I find in the program), would like to understand the process before then