It also helps to be able to compare
In my surf ski I can rotate all I want so I don't feel it as a distinct change between when I do and when I don't - there is nothing really to restrict me, aside from the width of the ski at the catch.
However, in my sea kayak (P&H Delphin), I have 2 paddling positions for my legs: with knees under the thigh braces or with knees in the center of the cockpit. There is a huge difference b/w the two in terms of forward paddling efficiency and ease. With knees to the side my rotation is limited mainly to twisting from the waist. This is how I paddle that boat most of the time, because I paddle it in moving and rough water mainly. However, when I get to a flat area and switch to knees in the center, my stroke all of a sudden feels "liberated" since now I don't have to twist so much any more but now I can rotate and get more leg action in. It's like someone turns a swith on or off ... My stroke is now longer (catch can go further forward, less arm paddling, legs now pump and move about more, etc.)
Case in point - if one only has the option to paddle with thighs/knees "locked", they can't really feel what unlocking your legs gives on top of that. Similar, if you don't have the restriction of braces (as in the ski) - you are always free to rotate so because you never feel restricted it's harder to appreciate the freedom as there is nothing more restrictive to compare to...