[pnako]

Because of the triple leverage, a 33.33% drop would entirely wipe out the fund. Back to $0. A 33% drop in a single day has never happened (1987 Black Monday was 22%) but it definitely does happen if you consider a longer time frame.

Hence why it's reset daily, as the other two responses explain.

And in case it's not been made clear enough: it's the very opposite of a sound investment. It's very much a risk management / trading instrument. It's something you would trade as a hedge or leg of a complex trade; not something you want to hold by itself.