Hi amikik, the big problem is what happens to $TVIX while you're waiting for the market to make a sudden move to the downside.
And then you have to ask yourself what would cause a sudden move to the downside. The Fed has made it clear they won't cause it for the next 3 months at least.
Maybe cascading bank failures in Europe ? Or Oil diving below $35 ?
Anyway -- to answer your question, TVIX is dependent on the VIX not the S&P 500. But I'd say that a quick 100 pt drop in the S&P 500 could cause TVIX to nearly double and a 200 pt drop would cause it to triple or even quadruple.