The deal with a "predictive market" is that people bet on a trend. With enough people doing so, they can predict, with high probability, what's going to happen.
for example, I followed the predictions regarding the recent election on intrade.com, (didn't bet,) and that proved to be pretty accurate.
TPCX is similiar to the stock market, without the financial risk and volatility, in that you invest in what you believe will be most successful. But, instead of trading stock in companies like you would on Nasdaq or NYSE, you trade stock in the outcome of specific policy issues or events; e.g. Who will Barack Obama name as FCC chairman?
If you invest wisely, i.e. you correctly predict the outcome, you are rewarded with an increase in your "net worth." You can trade in as many listings as you would like, as long as you keep in mind that your net worth can decrease just as it can increase…if you incorrectly predict the outcome of a market listing, you will forfeit however many TPC dollars you traded on that listing.