Back in mid-July, the iShares Edge MSCI Minimum Volatility USA ETF (USMV) - Get Report  was up over 12% for the year, more than double the return of the S&P 500 at the time. Fast forward to last Friday and the USMV is only up 5% year to date, only a percentage point ahead of the index.

Don't put too much weight into the USMV's second half selloff, it was trading ahead of itself early in the year anyway, said Robert Nestor, managing director and head of iShares Smart Beta Strategy at BlackRock (BLK) - Get Report  .

"Investors were very concerned about the risk in their portfolios given all the macro trends, so defensive areas were bid up a little earlier in the year and later they started to come back to the rest of the market," said Nestor. "It does not change the fundamental strategy there which is to reduce risk, not outperform."

The iShares Edge MSCI Min Vol USA exchange-traded fund seeks to track the investment results of an index composed of U.S. equities that taken as a whole have lower volatility characteristics relative to the broader U.S. equity market. Healthcare stocks are the leading sector in the USMV, taking up 19% of assets, followed by information technology and consumer staples at 15% each and financials at 10%.

Nestor said the iShares Edge MSCI USA Momentum Factor ETF (MTUM) - Get Report , up 4% year to date, has also been seeing inflows of late. In his view, the recent move towards momentum stocks reflects an increasing desire by investors to outperform and move away from the defensive stocks which led the market through July.

Two other smart-beta ETFs gaining a lot of attention, according to Nestor, are the iShares Edge MSCI Multifactor USA ETF (LRGF) - Get Report  , up 4% year to date, and the iShares Edge US Fixed Income Balanced Risk ETF (FIBR) - Get Report  , which is up 6.6% thus far in 2016.

The LRGF is the "reciprocal" of the USMV, according to Nestor, because it is designed to outperform as opposed to limit risk. Meanwhile, the FIBR's goal is to balance out duration and credit risk.

"Because of the rate-environment we have been in, most indices are dominated by duration risk, there is very little credit risk in those indices," said Nestor. "FIBR seeks to address that by offering a better risk/reward framework."