I use the "U" on bigger holes when I am not popping and they are shallower.
I watched a video once on youtube, Neswiper I believe, and he showed some holes with the "yellow eyes" because the grass was dead around the edge of a full plug, and eventually the whole plug.
His theory was when you stamp it back down, full plug, C, U, or flap cut, the water cannot get back down to replenish the roots because the now compacted dirt won't let it and the grass dies.
He suggested just chopping some lines with your digger all around the hole at the cut line perpendicular to the original cut to let the water back in.
I have been doing this for about 6 months and you know something, it seems to work.
I go back to the same park next door to me a lot when I just feel like swinging a bit and don't feel like driving far and I have been rechecking my heavily dug areas and...they look great!
"No Yellow eyes!" and no dead grass.