The body hair growth that people sometimes get on topical minoxidil is generally thought to be due to systemic absorbtion of a few percent of topically-applied minoxidil.
Other mechanisms are also possible. E.g., some minoxidil gets on the rest of your body when you take a shower. Its absorbtion gets enhanced because the skin is water-soaked ("hydrated") . It is even possible that enhancing growth of head hair somehow enhances growth of hair elsewhere.
Peter H. Proctor, PhD, MD