Ok, so, I still think my other answer is much more useful, but just to make @littleadv happy, here's how you can directly profit off of the percentage gain/loss phenominum:
You short a double to triple short leveraged ETF
The reason this works is because of the same thing that makes those 2x/3x ETFs suck:
Imagine you thought the S&P 500 was going to go down, and you were pretty sure about it. So, you bought an ETF that's double short that index. You figure the S&P 500 goes down, this ETF will go up by twice that amount.
Now, three months later, you check the markets and the S&P is indeed down 10%. Yay! You're ecstatic, you figure you made 20%! Right? Wrong. In fact, so wrong, that you could have actually lost money.
The reason is because of the phenomenon the poster asked about. Those double & triple short ETFs are double & triple the percentage loss or gain. So, if over the course of that three months the S&P had first gone up 20%, and then down 25% to end at 10% down, the double short ETF would have gone down 40% then up 50% to end at 10% down from the purchase price.
But, the numbers don't have to be as dramatic as that to make this work. The fact is each day the S&P will either go up or down a percentage, and the double short ETF will go down or up twice as much. This ends up having a ratcheting effect downward on that ETF's price over time.
So, if you think the index is going up, you can short the double short ETF, and put not only your long guess in your favor, but also this gain/loss ratcheting effect. You can end up being wrong about the index, and still making money.
I can't remember if this works on the 2x/3x long ETFs, but IIRC it does, just not quite as well as on the short ETFs.
I've never actually tried this myself.