In my mind, it's a very extreme form of commitment reserved for out of the ordinary scenarios. Not something I would recommend telling a boss on a regular basis, as it robs the term of its degree of commitment.
You might bend over backwards to win a key client from a competitor, but you wouldn't bend over backwards to get the weekly report in on time.
Even then, to use the phrase is indicative that you really need something to happen and that the consequences for your job/reputation/well being might be extreme if you couldn't. One typically bends over backwards not just for someone in a higher position than oneself, but also when the person you're doing it for shows little regard for you, because they know they're in control due to their station.
People who bend over backwards often assume a submissive role, which is not to be mistaken with there mere seniority that a boss would have over you at work.