What makes you think you will only lose on commission during a whipsaw market? You'll pay slippage as well, and may end up buying the market back for more than you sold it at (if the 50 day MA is still rising during your whipsaws). There have been 34 days in the last year where the market went through the 50 day MA. And that's only counting each day once - in reality it may have whipsawed half a dozen times in one day. Let's say it whipsaws on average 3 times during days when it crosses the MA. So you will be making 102 transactions in your portfolio. Assume a 3 cent spread on the SPY, plus another cent in commissions - you would have paid 3.7% of your portfolio in transactions costs. You would also have received less in dividend payments - let's say you are out of the market 1/3 of the time, you would lose another 0.55% in missed dividends. You would also lose the reinvestment impact of those missed dividends. Assume an average buy and hold return of the 10-year yield plus 2.5% i.e. 7.25% right now - you would have returned 7.25-3.7-0.55=3%. So your strategy would capture only 41% of the buy and hold return. And this assumes that you can always buy the 50 day MA cross back at the same price you sold it at - in reality, you will often be buying it back higher. Note that this poor performance covers a period where there was a rampant bull market. Finally, you will pay a lot more in capital gains taxes.