Still, Ames said the strategies resonate with investors shell-shocked by the 2008 credit meltdown, which changed their entire mindset on the market. Investors are paying attention to risk, not just return. They want to limit volatility and protect themselves on the downside.

“Risk-adjusted performance is now an important factor in investors’ minds, more than it was five or 10 years ago,” he said. “Investors are very aware of risk—they want to minimize risk and maximize returns. In the past it was all about maximizing returns only.”

Invesco PowerShares was also “fortunate” to launch SPLV in the spring of 2011 right before the summer market plunge on European debt fears. “The ETF, based on the underlying index, did what it was supposed to do, and the investment community noticed,” Ames said.

Investors also like the fund’s low fees – it has an expense ratio of 0.25%.

Currently, SPLV has some tilts to traditionally defensive sectors that are very different from the S&P 500. For example, it has 28.7% in the consumer staples sector and 31.7% in utilities.

“The strategy can dynamically rotate into other sectors based on volatility,” Ames said.

‘The low-volatility approach has delivered’

Invesco PowerShares also oversees similar ETFs that invest in international stocks: PowerShares S&P Emerging Markets Low Volatility Portfolio (NYSEArca: EELV) and PowerShares S&P International Developed Low Volatility Portfolio (NYSEArca: IDLV).

Other low-volatility ETFs for U.S. stocks include iShares MSCI USA Minimum Volatility Fund (NYSEArca: USMV) and Russell 100 Low Volatility ETF (NYSEArca: LVOL). For low-vol international stocks, investors can take a look at iShares MSCI All Country World Minimum Volatility (NYSEArca: ACWV).

“For investors determined to stick with stocks but not loving the latest Maalox-inducing moments, plugging into low-volatility ETFs can be an intriguing solution,” said YCharts editor Carla Fried in a recent article for Forbes.com.

“Clearly, the low-volatility approach has delivered in down markets, as you’d expect. But in up markets … a low-volatility portfolio isn’t going to be the pacesetter,” Fried wrote.

PowerShares S&P Low Volatility Portfolio