There are many short periods where TLT-SPY has positive correlation. If both go up then anyway we are happy. If both go down like 2013 for a short time, then it is not so good. If you analyze the correlation, then you see that it has it’s negative lows always if there is a market correction. So, in fact this means that if the market crashes investors sell SPY and go into the safe haven ETF TLT. As long as it is like this the strategy works. During low volatility calm bull markets, there is no problem if the correlation fluctuates sometimes towards zero.
