Hatsu, Rei, Ho
"Hatsu 発 means to arise, to give birth, to reveal what is hidden or to emit. Rei 霊 means spirit, soul or inconceivable spiritual ability. Ho 法 means dharma, ultimate truth or method." -Master Stiene
For many people, the mind runs a little ahead of, or sometimes a little bit behind, what the body is doing, but to unify body & mind means that there is no more "lag time" between an action & a thought
So when the body is sitting in Zazen or Mokuso, for example, in a straight yet relaxed posture, breathing deep & slow, the mind is also completely sitting in Zazen or Mokuso &c.
And when the body is chopping wood, or walking up a mountain, or playing the bamboo flute, so is the mind -- indistinguishable, like water poured into water, fire thrown into fire!
For me, this complete unification of body & mind right here now is the wonder of all wonders, the marvelous Dharma secret of the whole Universe.
One of the delightful side-effects of body-mind unification is that rude & high handed behavior can no longer disturb you at all. It makes to laugh, as the French say.
Another good side effect is that you can easily stop your mind from tormenting you with images of all your past humiliations & social gaffes.
"Il faut rire avant d'être heureux, de peur de mourir sans avoir ri." -Jean de La Bruyère