Fed Chairman Bernanke’s little stock rally, which was initiated when he announced on 60 Minutes that the Fed was “electronically’ printing dollars, has apparently come to an end (at least for now). As a currency trader and market observer, you’ll want to know why and the reason has to do with the most famous currency pair you’ve never heard of. Ready? It’s called…

S&P/USD

That the dollar moves inversely to riskier assets like stocks (and commodities as well) isn’t a matter of some kind of mystical correlation that can somehow “break.” The key to knowing this is to understand what really is happening when stocks (or commodities) appreciate, which is easy to see once you accept the existence of S&P/USD.

Look at it this way; when stocks go up, what are they going up against? The dollar of course. The same for goes for commodities because those are priced in dollars all over the world. Well, if these riskier assets have gone up, isn’t it than also true to say the dollar has depreciated. Absolutely. So when the market is buying risk, the dollar will tend to fall as a matter of due course.

What’s really interesting is that in the rare instance (like now) when the Fed is actively trying to devalue the dollar (because the economy is faced was the more serious threat of deflation), the Fed can create a stock rally just as Bernanke intended to do by admitting to the electronic printing of dollars on national television. Why? Because if you are convinced the dollar is going to depreciate, doesn’t that also mean that anything you can buy with it is going to be more expensive? Of course. And the convinced you become that the prices of liquid assets (like stocks and commodities) are going to rise, the more inclined you will be to buy them.

So what’s really happening in the current environment is that the Fed, through its actions and comments on the dollar (and on deflation/inflation which is the same thing) is pushing the market for riskier asset classes up and down. Let’s look at some recent action.

Bernanke’s rally pretty much fizzled out in the middle of May, but it got a temporary boost (about 4%) in the week following the last meeting on June 24 when members removed the deflationary concerns which had appeared in the April 29th statement. But then something interesting happened.

On June 30, San Francisco Federal Reserve Bank President Janet Yellen expressed some deep concerns about deflation. “I’ll put my cards on the table right away,” she said. “I think the predominant risk is that inflation will be too low, not too high, over the next several years.”

Now, remember what we said; if the market is convinced the Fed is depreciating (creating inflation), riskier assets are going to be bought because inflation means their prices are going to rise (at least nominally). No one wants to hold cash if the potential for inflation is on the horizon, because cash is guaranteed to lose value in an inflationary environment.

Conversely, if inflation really isn’t a threat and the possibility of deflation exists, there’s no reason whatsoever to buy riskier assets. Afterall, your cash is guaranteed to increase in value in a deflationary environment because prices are going down!

In fact, in a deflationary environment you make more money simply by being a “lender” (keeping for money in the bank or in Treasuries). A little arithmetic will show you how this works.

Your real (inflation adjusted) rate of interest is equal to the nominal rate (what the bank is paying you) minus the rate of inflation or R = N – i.

Let’s say inflation is 3% and your bank is paying you a nominal 4% on your deposit. Your real (R) interest rate is the Nominal (N) rate minus inflation or R = 4% – 3% which equals 1%. In real terms you’ve only earned 1% on the money you have in the bank

Now, let’s say there’s deflation (negative inflation) of -2% for the year and your bank is paying you a nominal rate of 3% on your deposit. What’s the real rate of interest you’ve earned?

R = N – I or R = 3 – (-2) = 5%. In real terms, you’ve earned 5% on your deposit!

Now, if I’m guaranteed to earn a risk-free return of say 5%, and I think prices are going down, why in the world would I buy a riskier asset? In the hope that someday its price might go up? Hope is a bad rational for making a trade.

So, Ms. Yellen, with her deflationary concerns, helped kill the rally. She wasn’t alone however; the Fed, with its expert manipulation of the media via the pundits seen all over Bloomberg, started so make plenty of deflationary noises around the middle of May.

As a trader, there are times when you can put this to very good use for longer term trends (which really is where the money is made). If the Fed does start talking about inflation and making noises like it might raise interest rates, do yourself a favor and sell the dollar as you buy riskier assets like stocks and commodities.  But if they continue to talk about the risks of deflation, there’s little reason to sell the dollar or to buy riskier assets. Buy the dollar at that point because it will be in demand.

BTW, Thursday’s weak market action off the surprisingly good unemployment numbers, along with the subsequent decline of S&P futures overnight, bodes very poorly for stocks and very good for the dollar. I can hardly think of a more bearish sign for stocks than the market failing to rally on good news. Afterall, it the market can’t rise when the news is good…