This grocer isn't looking very fresh right now.

Shares of grocery chain Kroger Co. (KR) - Get Report are under serious pressure Friday, down more than 8% before noon on the heels of the firm's second-quarter earnings call.

Kroger posted a profit of 39 cents per share for the quarter, right in line with analysts' expectations, on average. But the firm ended its practice of providing Wall Street with long-term guidance, a move that investors are seeing as a bearish sign as grocery chains struggle with even more competition, including the likes of Amazon.com Inc. (AMZN) - Get Report .

Grocery is already an industry that comes with paper-thin margins and stiff competition. The addition of a disruptive force like Amazon, through its recent acquisition of Whole Foods, makes things worse. And Kroger's guidance policy shift is being read as confirmation that grocery margins are indeed facing an uncertain future ahead.

But a more immediate problem for Kroger investors might be the price action. So, we're turning to the chart for a technical look at how to decipher and trade it:

Without a doubt, 2017 has been a challenging year for shares of Kroger. This stock has shed almost 40% of its market value since the calendar flipped to January, underperforming the rest of the market by a massive degree. And that bearish momentum isn't showing any signs of a turnaround in September.

Shares are trading below that crucial $21 level intraday today, but the sell signal isn't confirmed until shares close below that line in the sand.

Kroger is currently forming a descending triangle pattern, a bearish continuation pattern that opens the door to more possible downside ahead. The setup is formed by horizontal support down below shares at $21, a price floor that's been in place since mid-June, along with downtrending resistance to the top-side.

Basically, as Kroger pinballs in between those two technically important price levels, this stock has been getting squeezed closer and closer to a breakdown through its $21 price floor. If and when that happens, we've got a new sell signal for KR.

What makes that $21 level in particular so significant for Kroger? It all comes down to buyers and sellers. The $21 support level in KR is a place where there has been an excess of demand for shares; in other words, it's a spot where buyers have been more eager to step in and buy shares than sellers have been to take gains. That's what makes a breakdown below $21 so significant -- the move means that sellers are finally strong enough to absorb all of the excess demand at that price level.

Sellers are clearly in control of Kroger's price action right now. Buyer beware.

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This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.