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Why buying an inverse ETF does not give same results as shorting the ETF
The most fundamental answer is that when you short a stock (or an ETF), you short a specific number of shares on a specific day, and you probably don't adjust this much as the price wobbles goes up and down. But an inverse fund is not tied to a specific start date, like your own transaction is.  It adjusts on an ongoing basis to maintain its full specified leverage at all times.  If the underlying index goes up, it has to effectively "buy in" because its collateral is no longer sufficient to support its open position.  On the other hand, if the underlying index goes down, that frees up collateral which is used to effectively short-sell more of the underlying.  So by design it will buy high and sell low, and so any volatility will pump money out of the fund. I say "effectively" because inverse funds use derivatives and contracts, rather than actually shorting the underlying security.   Which brings up the less fundamental issue.  These derivatives and contracts are relatively opaque; the counter-parties are in it for their own benefit, not yours; and the people who run the fund get their expenses regardless of how you do, and they are hard for you to monitor.  This is a hazardous combination.