Footnote IX leads to a study published in the journal "Fluoride" by Elsair, J, in 1980. Scroll down until you get to pages 30-38. http://www.fluorideresearch.org/131/files/FJ1980_v13_n1_p001-045.pdf
I wouldn't say it describes the "actual mechanism" by which boron detoxifies excess fluoride, except that boron promotes healthy levels of calcium and phosphorus, and those two bind the fluoride for excretion. The article also makes reference to a similar study done in 1977 by Baer et al. That one is not so easy to track down since it was published in German(y).