What happens when an alkylborane is treated with acetic acid?

In the hydroboration reactions of alkenes I've seen so far, they were always followed by oxidation with $$\ce{H2O2/NaOH}$$ (to yield alcohols).

Today I found a peculiar reaction in which acetic acid-d1 was added after hydroboration to give a deuterated product:

I don't quite understand what exactly happens after the hydroboration part. Can someone enlighten me here?