Here is something from my 'private stock.'
This is the picture of a quiet flight to quality.
If you must trade, buy strength and sell weakness, and not the other way around when driven by greed and fear.
But for almost everyone, it is better to see the trend and ride its crest, perhaps hedging a little at the extremes, while the fundamentals that created it are intact.
What are the fundamentals driving this phenomenon? Keep Jesse's Paradox in mind.
And if you have to rely on something wonkish, with the trappings of an economic theory, then an eye to negative interest rates on the ten year bond is not bad, provided that you can find a measure of price inflation that has not been fouled by official corruption.