One can short TZA. . .I've done it.
Your study of TZA only went one way. Hence, what do you get if you go long and short? Here, I'm not suggesting going long TNA, but rather selling TZA, i.e., shorting it.
For example, sell it short between 3.12.2009 and 6.16.2009.
That's a huge move lower.
(One can also, of course, go long or short TNA.)