Rolling
I call "rolling" the move that ensures correct twist of arrow in its long axis, so that its nock can be immediatelly locked on the bowstring. The easiest way looks like this:
1) Take the end of arrow between your thumb and index finger
2) Roll the arrow on the index finger by your thumb.
The arrow rolls, and you feel the position of nock because it is a bit flat. You can feel how the groove in the nock is twisted (thanks to its flatness) with eyes close. If it is not as it should be, you can easilly change it by rolling.
It is never necessary to roll more than a quarter turn. And with suitably modified nock even less, because V-shaped groove easily quides the arrow to the right position even if it is not rolled perfectly. It is often enough to just push the arrow a bit against index finger, and with a slight movement youst lean it on one side or the other.
The arrow may be rolled with thumb even if you hold it between other fingers, but you have to hold it softly. There are some holdings it is not possible to roll at all, or almost at all. For example whenb holding between index and mioddle fingers with the arrow moved as near as possible to the palm. Rolling is also not performed well when holding arrow in the space between straightened index and middle finger.
In these ocassions you are able only lean the arrow a bit by your thumb. The rest of necessary roll is achieved by twisting whole back hand (with the arrow). With nocks suitably modified only the slight twist is enough. But always it is necessary to feel the position of arrow by your thumb, so that you know how to twist the hand.
Rolling either with thumb or whole hand is done usually quickly during nocking, and if done correctly, it means no loss of time. But it is necessary to practice enough. If you are sure with rolling, it will enable lightning-fast arrow nocking.