This Japanese expression has two basic meanings. The literal meaning is to break one's bone(s), as when falling from a height. The figurative meaning is "to try harder." That is, to try so hard that one's bones break.
Undertaking bonebreaking work is a core value in a classical Japanesedojo. Passage through such an ordeal presents the karateka with new insights into his or her kokoro, or heart-mind-spirit. These insights often serve to diminish the strangle-hold the ego had held on the mind prior to the ordeal. Contrary to popular belief, diminishment of the ego does not render a person ineffectual in the world. In fact, the smaller the purchase your ego has on your life, the more able you are to access highly functional energies and insights that a too-strong ego would deny. A person with a strong bond to kokoro places confidence in something beyond the small self of the personality. This bond provides the strength to prevail when other's, more rooted in their own egos, fall by the wayside.
Kaicho has often spoke of this aspect of Seido training, the aspect of managing the ego. It is Kaicho's belief that this encounter with one's own ego (defenses, projections, delusions) is the fundamental "fight" that the true karateka wages, and that hard training is the path to ultimate victory. Thus, we in Seido don't "train for" some tournament of upcoming event, as much as we "train through" the continual, "bone rattling" battles we have with the ego.
See you in the dojo.