Here's what is going on with bank stocks.

Bank stocks are lagging the market at a moment when they should be on fire. To figure out why, Jim Cramer spoke with Ed Ponsi, managing director of Barchetta Capital Management, and contributor to RealMoney.com in Cramer's "Off The Charts" segment of Mad Money.

Ponsi started by comparing the S&P Select Financial SPDR ETF (XLF - Get Report) , a bellwether for the financial sector, in red, with the SPDR S&P 500 ETF (SPY - Get Report) , in green.

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Because there are so many large bank stocks in the S&P, Ponsi says it would be pretty unusual for the financials to diverge from the rest of the market in terms of their performance, yet this is exactly what's happening right now. The S&P as a whole is surging while the financial index is not.

These two indices diverged dramatically about three weeks ago on May 15. Since then, the S&P has made a series of higher highs, breaking out to new all-time highs, while the financial index has had a series of lower highs. Ponsi says it's possible that investors are selling their bank stocks in order to pour money into sexier sectors like software, semiconductors, and tech titans like Amazon (AMZN - Get Report) and Alphabet (GOOGL - Get Report) .

Some of this may also be driven by the technicals. An individual chart of the XLF, the S&P Financial ETF shows one of the most negative patterns around, the dreaded head and shoulders formation.

Right now the XLF is trading at $23 and change, and if it breaks down below the neckline of this pattern at $22.90, less than half a buck below where it's currently trading, Ponsi believes it could quickly fall all the way to $21, down roughly 10%.

Ponsi says it's time to be wary, but not to panic. The financial ETF looked like it was going to break down back on April 13, point A on this chart, but then it held. The same thing happened again on May 18, point B. Once again, the XLF was bailed out by dip buyers who took advantage of the broader stock market's weakness.

That's Ponsi's point. While the financials don't seem to be getting much lift from the strength of the stock market, it's certainly kept them from breaking down. Just look at this chart of the SPY-the big S&P 500 ETF.

On April 13, point A, the market bottomed after a brief decline and then roared higher. On May 18, the same thing happened. And Ponsi thinks that's what backstopped the decline. Unfortunately, he says the strength in the broader market is probably the only reason why the bank stocks haven't broken down yet. 

At the heart of banks' problem is what's happening with 10-year U.S. Treasuries. Banks do better when interest rates go higher, so next week's expected Fed hike should be good news. Sadly for the financials, bond prices have been going up and thus bond yields have been getting crushed. It happened again today. The yield on the 10-year is now almost back to where it was before the election, when everybody got a lot more bullish and everything took off.

Some people might read this action as suggesting that the economy's weaker than it seems and maybe, just maybe, the Federal Reserve might not tighten next week That seems a unlikely with the economy at nearly full employment. The other possibility is that there's a worldwide shortage of safe bonds with decent yields -- rates in much of Europe are still negative -- which causes investors from overseas to swap into U.S. Treasuries. Either way, though, lower rates are bad news for the banks.

Given Ponsi's interpretations, caution around the financials is warranted. But all is likely to be forgiven if the Fed tightens next week.

Shares of JPMorgan (JPM - Get Report) were rising by 0.2% early Wednesday while Goldman Sachs (GS - Get Report) slipped slightly.

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