Thanks for the tag @Rygel.
@Tradingplc @End-Of-Eternity - the thing to keep in mind about inverse ETFs is that they usually rebalance daily. Simply put, this means that each day is treated as a separate event, so when held for longer periods - the return is not what you expect.
For example, let’s consider an ETF like ‘SPY’ that tracks the S&P 500 index and an inverse fund like ticker ‘SH’ that returns -1x (or inverse) the daily performance of the S&P 500 index.
Essentially, ‘SH’ does the opposite of what ‘SPY’ does. We will assume that they both start at a price of GBP 100.
2 Scenarios to Outline the Difference 
1. Volatile moves.
During volatile moves in SPY, it’s likely that the the inverse product (whether -1x, -2x, or -3x) will underperform compared to what you expect:
You can see that after 1 day: you get what you expect (you made 10%, since ‘SPY’ lost 10%). But by day 3, you have lost over 8% (instead of being at 0% like ‘SPY’).
2. Trending down market.
It’s likely that it will overperform compared to what you expect:
You can see that after 1 day: you get what you expect (you made 10%). But by day 3, you have made over 33% (instead of the expected 27% from the decline in ‘SPY’).
So briefly - for longer periods, short selling will give you the expected returns, while for a single day, the inverse ETF “does its job.” The difference in the inverse ETF is cause by ‘daily compounding.’