"helping your uncle jack, off a horse" is not proper English. Ever. Commas are only used to connect two independent clauses of a sentence, clauses. "Off a horse" is an incomplete clause.

If you are helping your Uncle Jack, you are helping him do something and, without the comma, it makes sense. I think that sentence is one of the most relevant expositions about the travails of English that there is. English does not clearly identify the object and subject, while some languages have suffixes or rigid syntactic order to avoid confusion. For us, capitalisation is an easy get-out.

I commend to you:
http://grammar.ccc.commnet.edu/grammar/commas.htm