OK, ready to get your nerd on?
Bone meal is exactly what it sounds like, ground up bones. It has some nitrogen in it, but not a lot, and is mostly a source of calcium and phosphate. Indeed, bone meal can be approximated as Ca3(PO4)2 and also Ca10(PO4)6(OH)2. By weight, calcium makes up a bit less than 60% and phosphorous about 30%, with the rest being nitrogen, magnesium, salt, and other assorted gunk. When the calcium salts are in an alkaline state, they're relatively stable, and can only be broken down by mycorrhizal action; certain fungi are able to get at that phosphorous, but otherwise, it might as well be part of rock, because... well, its part of a rock. When you lower the pH, though, you have a ratio of H / OH that favours H+, that is, more free H+ ions.
Once you expose those calcium salts to an acid, they tend to break down pretty quickly -- for example, if you have, say, nitric acid:
Ca3(PO4)2 + 6HNO3 + 12H2O --> 2H3PO4 + 3Ca(NO3)2 + 12H2O, and that H3PO4 is quite accessible to plants in water. (A similar reaction would occur with any acid -- which exists if pH < 7.0). Similarly, the Ca10etc gets broken down, allowing that PO4 to be gobbled up by plants.
Now, in a planted tank, do we really worry about pH affecting this? Chances are, we're pumping in CO2, and that'll produce carbonic acid OC(OH)2, which will nicely substitute for the HNO3. Even there, especially in the substrate, microbial activity tends to lower pH.