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"What's working?"

That's the question investors should be asking right now, as markets struggle to find traction in December. Simply put, the stocks that were leading the broad market higher as recently as September suffered a painful about-face when October's correction came knocking.

That's been compounded by a lackluster start to 2018. As of Monday's close, the S&P 500 was set to finish its worst year since 2008. And while U.S. equities are bouncing early Tuesday, that bounce has been fading as we extend into the afternoon hours.

Now, with a rotation in leadership underway, doubling down on the stocks that are still working in this corrective environment could mean the difference between ending 2018 on a high note and finishing the year in negative territory.

One of the stocks that's working right now is UnitedHealth Group Inc. (UNH) , the big-cap diversified health care company.

UnitedHealth isn't just looking stronger than the rest of the market right now - it's also signaling an outright buy as shares bounce higher today.

The good news? You don't need to be a trading expert to decipher the buy signal in shares. Instead, the setup is about as simple as they get.

To figure out what's going on with UnitedHealth, we're turning to the chart for a technical look:

The uptrend in UnitedHealth has been pretty unmistakable this year. While the S&P is struggling to get back to breakeven year-to-date, shares of UnitedHealth have actually been in rally-mode, managing to move 22% higher since the calendar flipped to January.

UnitedHealth entered an extremely well-defined uptrending channel back in late Spring. Since then, every test of the bottom of that trend channel has provided investors with a high-probability buying opportunity before this stock's next up-move. That relationship held up during the October correction in shares of UnitedHealth, and it's continuing to hold this week as shares bounce off of support for a third time in as many months.

Relative strength continues to be one of the most important side-indicators to keep an eye on for any stock in your portfolio. The fact that relative strength actually began trending higher in October is telling. While UnitedHealth has been outperforming the rest of the S&P 500 all year long, that outperformance is actually accelerating now as pressure increases on the rest of the broad market.

If you decide to buy the bounce in UnitedHealth this week, risk management still remains key. It makes sense to park a protective stop on the other side of the $259 level that currently marks the bottom of UnitedHealth's trend channel.

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