Math & Science ⇒ What do \mathaccentV and \Umathaccent do?

[user49915]

What do these two commands (\mathaccentV for pdflatex and \Umathaccent for {xe|lua}latex) do exactly and how to use them (or how are they used)? As an example, could anyone give an example of how to use them to construct right arrows above math symbols, such as, e.g., a⃗ (except that the small letter a should be a math one). You can assume that the package mathtools is always loaded, and that in the UTF-8-case, unicode-math is loaded.