>> when is the problem <<
Yes, and with the 2X, 3X short ETFs the timing would have to be near perfect. Get it wrong and your money evaporates FAST. They also 'erode' over time due to the nature of their structure, having to 'reset' periodically as the futures contracts expire and have to be replaced (or something like that). Definitely not a vehicle to hold for any length of time, you have to be in and out quick. Personally, I would forget about 2X, 3X, especially on the short side. Just my 2 cents :o)
---