We explicetly try do nikyo without giving pain to the partner.
... nikkyo is essentially a general shape of energy... a "gathering" movement of energy, as though you are scooping up armsfull of tall grass and bringing them into your belly.
... a certain kind of spiral movement.
This sounds familiar.
may give a little idea of how it works. Even if the nikyo omote shown here is done with contact at the shoulder of tori it works the same way when you do it more related to the form of ikkyo omote.
As far as I understand it by now, the "gathering" or closing in nikyo omote is done by closing kua, shoulder nests and spine.
The "spiral movement" or opening in nikyo ura is done by opening shoulder nest, spine and kua.
Whether nikyo is worked by pain or "only" by contact without using or producing pain to my experience is not a question of aikikai, yoshinkan, ... .
Our shihan is a well known teacher of aikikai hombu. no pain
The aikikai shihan in charge for germany employs pain doing nikyo and emphasize it's use a lot.