I think that's definitely good enough. You see an important metric in gauging _good_ answers is also how succinctly you've been able to express yourself. It is not merely good enough to have done something cool. Especially for YC.

...of groups good enough to make it to interviews, more than half blow the application.

Indeed not surprising. Rephrase what you're trying to say so it brings out why this was so brilliant a hack in the least number of words possible. Rewrite it again.

We're looking for people who like to beat the system. So if the answer to this question is good enough, it will make me go back and take a second look at an application that otherwise seemed unpromising.

It is definitely one of the most important questions in the application. Think of other things you have done that might seem trivial to you but might be really brilliant. Talk to your friends. They might help.

In any case, best wishes for your YC Application.

* Excerpts from http://ycombinator.com/howtoapply.html