It's not unusual for FAZ to swing that much on any given day. It hasn't lately, volatility has been low.
It's interesting to note that FAZ this morning is 2% lower than it was on Feb. 1, the last time the index was at 883. So it lost 2% during three weeks of low volatility.
The volatility index increased 20% yesterday, so we can expect the volatility component in FAZ's decay to accelerate.
My point is: I think it is probably a mistake to invest in FAZ or FAS based upon a bull or bear viewpoint.