Myself, I consider it being a cushion, pillow, speedbump, poof or whatever, when the back line (often more toward the tail) is not flat or hollow, but rising. A banana with the inside hollow end up placed in the back is a nice concave sweep (hollow), the Welsummers do not have this is, it more straight/flat and sometimes slight sloping toward the tail. Then the tail quickly and abrupt emerges, held very high without any fluff (down) showing in that break. Back to the banana, if it is bold end up, its a bump and that is what I call a "pillow"
Here some pics of my buff cochins with big pillows (and should have), followed by some Welsummers that IMO have good backlines. Now keep in mind when the hen goes to sctrach the floor or is very relaxed with tail down, then she can look worse than it actually is. Always good to have a show coop style box in your barn to place them in and observe from your own comfortable height. The bird must be able to pose herself and then you can better judge her shape. There are many other breeds prone to this and for some it is a must have in sop, I do not find the Welsummers have this problem very much in general and only see it more in the German type Welsummer, they have them much more riding upward, from the beginning of the back toward to the tail. The tails are then lower set and have much less of a noticeable transition between back and tail and gives more room for a "poof" or "cushion"
I hope this explains it a bit better for you granthamkk.
I specifically kept a roo with a lot of red in his chest for this very reason!! The other thing I'm wanting to breed for is a more flat back, my last roo stood up way too tall.I have peppering in my flock as well. Piet gave advise on choosing a male with more red in his chest to balance out the peppering. I'm on the lookout for offspring with red chests now!