Further, you can tell us what the business is, and what is compelling about it, without giving up the secret sauce. You can even mention the secret sauce without giving up the secret.
EG, if your startup was google: We're building a revolutionary search engine using the social proof inherent in the web to give people results that are far more relevant than Yahoo and Inktomi. There, did I give away the page rank algorithm? No, but I did reveal the compelling advantage that google had: they figured out how to derive social proof from the web... which at the time was unheard of.
Even if that's too revealing.... at least talk about your technology stack. If you write a 500 word essay and the only mentions of technology is that you use "rails/node.js" -- something at first blush seems like not a choice but a pair of choices-- you're being evasive one something you have no reason to be evasive on. "We're using rails to host the primary web app, and node.js to run a really nice realtime updating system, blah blah blah."
You can talk about that... and you are giving candidates an opportunity to know what you're like based on your technology choices and how you talk about technology.