Kitsune have something called a Hoshi no tama, star ball or kitsune ball. They are white and/or gold orbs. The orb is like a kitsune's life force so they hide it, usually in their mouths or on their tails. If someone manages to get a hold of the orb, they have control over the kitsune.
What is the extent of that control? Could one keep a kitsune until they die and the kitsune reclaim the ball?
If the fox has a "ball" in it's mouth it is actually the female incarnation of a fox. The male fox incarnation has a parchment or key in his mouth. The "ball" is actually the pill of immortality. The parchment/key is the contract a mortal agrees to with the foxes. Since foxes are shapeshifters, they can appear as female or male. This duality is congruent with the principles of Yin/Yang. You don't control foxes unless you are a Taoist/Shinto fox master. You have an agreement with them. This agreement allows the mortal the opportunity to develop their soul in this life.
The Center For Traditional Taoist Studies published a 3 volume set on Fox lore called: The Tao of Celestial Foxes-The Way to Immortality. It has an extensive bibliography to say the least
