Some may hold off on exchange traded funds (ETFs) in favor of a different investment tool, HOLDRS, which are showing some good performances in the sub-sector of oil refiners. But how does one use them?

HOLDRS, first launched a decade ago, are static portfolios. This means that if mergers occur between components, or if a component fails, then the weightings are shifted around among the surviving firms, remarks Matthew Hougan for IndexUniverse. This can result in volatility and extremely heavy concentrations.

Another quirk of HOLDRS is that an investor has to trade in 100-share blocks to get into the action.

Investors may also redeem shares for underlying holdings in HOLDRS. Someone may go to a brokerage, with a fee, and swap the 100 shares or so with equal value in underlying shares of the exchange.

Still, March ETF fund flows data showed that Oil Services HOLDRs (OIH) was in the top 10 and it raked in $547 million for the month. It should be noted that its weightings have a peculiar characteristic: only 16 components. This is far fewer than its peers.