The momentum factor has been topping value this year and that has helped the iShares MSCI USA Momentum Factor ETF (NYSEArca: MTUM) to a year-to-date gain of 7.6%.

MTUM, which tracks large- and mid-cap U.S. stocks with relatively high price momentum. The underlying MSCI USA Momentum Index calculates the ratio of each stock’s price returns over the trailing 13 and seven months against volatility over the past three years. Companies are then weighted by their risk-adjusted momentum. [Expanding Economic Conditions Should Favor Growth Stock ETFs]

MTUM’s underlying index reconstitutes holdings only twice per year in May and November, and includes a broad buffer to diminish turnover and potential transaction costs.

Potential investors should be aware that the momentum strategy typically works well under sustained market rallies and could breakdown during volatile conditions. For instance, the underlying benchmark underperformed the MSCI USA Index by 3.8% during the 2008 financial crisis.

“Momentum is different from the volatility of a stock. In simple terms, momentum is the rate of acceleration of a security’s price or volume. It’s very helpful, even more than moving averages, when used in combination with another indicator to estimate the future movement of a share,” according to Market Realist.

Moreover, since momentum strategies can overweight riskier stocks, the ETF could could underperform during another correction. Since defensive stocks typically do better during volatile conditions, the momentum strategy could load up on conservative picks and miss out on the initial recovery in riskier assets.