If you do something off your own bat, you don't require assistance. If you use the incorrect phrase "off your own back", you definitely need help.
Picture: Getty Images/ Robert Cianflone
The Macquarie Dictionary defines "off one's own bat" as meaning "independently; without prompting or assistance". It also notes how the term came about:
from cricket, with reference to a score made off the bat, as opposed to byes, wides, etc.
Writing "off your own back" is thus incorrect (and confusing). It's a common enough mistake that it's possible that at some point in the future this phrase will become the standard version. But given cricket's ongoing popularity, I won't be holding my breath. In the meantime, be sure to use "bat", not "back", when you deploy this phrase. Accuracy matters.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.