One of my favorite recurring observations about financial markets is that the classic concept of "stocks vs bonds" doesn't always work.  That's the one where one side of the stock/bond equation explains its movement by citing big moves in the other.  Rarely was this more comical than in recent months when stock analysts blamed rising bond yields essentially any time stocks moved lower--just never you mind that bond yields spiked high/faster in late 2013 without any meaningful stock drama.  

This time around, bonds are indeed at least somewhat interested in what's going on with stocks.  In fact, bonds are typically always willing to rally if stocks are selling-off quickly enough.  With major equities indices crossing below some key technical thresholds today (and quickly approaching February's flash crash levels), there is definitely some safe-haven money coming into the bond market.  In other words, the "classic concept" was intact today insofar as stock market weakness helped bonds rally.

But while the correlation was sound, the magnitude left something to be desired as far as bond bulls are concerned.  A fairly big move lower in stocks only coaxed a modest amount of buying out of bond traders.  This begs the question: what will bonds do if stocks hold steady or improve from here?  What would they have done today without the stock sell-off?  At the very least, we can assume the answer wouldn't start with bonds continuing to rally.  As such, today's gains feel fragile until further notice.