It works if you have a deep pocket to withstand the margin call in worst case. And the other problem is that it's hard to borrow those shares to short.
Good. Shorting FAZ is much better. Why? 2 reasons: (1) More likely market will go up in long-run (2) When FAZ goes up, VIX goes up and FAZ decays faster. This make it a safer short. One strategy is to short FAS:FAS at a 1:2 ratio ($ amount), or 2:3 ratio......