There are many different techniques for Yoni massage.
Some practitioners do a Yoni massage as a mechanical massaging of the tissues, e.g. clock wise, or back and forth. The idea is that tension is released that way and the Yoni heals and becomes more sensitive. Personally, I think that a Yoni massage is not as simple as that and the Yoni needs a lot more to change.
There is also a lot of advice, on Tantric massage for women, on how to touch the Yoni in different ways as part of a Yoni massage, e.g. fingers spread like a fork, pinching, beckoning motion, etc. The emphasis is on creating a wide range of pleasant sexual sensations in the Yoni, and sometimes a form of stimulating it in a deeper and more pleasurable manner than usual.
In a real Tantric massage, the next level from that would be where not only the kind of stimulation is different, but also a different kind of consciousness and emotion is practiced by the women purposefully during the Yoni massage. A different kind of consciousness and emotion can also be practised by the man who is giving it. As usual, the real potential is not just in the physical, but in combining it with the rest of ourselves as human beings – our minds, our hearts, our consciousness.
Techniques exist to do that, and they complement the physical technique that would happen at the same time to provide sensations of sexual energy. Then the Yoni has a real chance to heal, to rewire, to change, to become more sensitive and carry more kundalini for the woman and for the man.