AllianceBernstein Unconstrained aims to deliver positive returns every year. To accomplish that, the fund managers hold a mix of high-quality and below-investment grade bonds. They tilt the mix as conditions change. These days the fund is shifting away from safe government bonds and toward corporate issues that can prosper as the economy grows. The fund's low-quality issues have recorded gains in recent months. Portfolio manager Michael Mon holds some assets in emerging markets, which should gain as investors decide to take on more risk. Mon owns a stake in Mexican securities. "Those yield more than U.S. Treasuries, and the risks are limited," he says. Pimco Unconstrained ranks as the largest fund in the group with $22 billion in assets. Some investors have raced to the fund partly because they believed it could outdo Pimco Total Return ( PTTAX), which has boasted a stellar record under longtime manager Bill Gross. While both funds rely on the same research, the unconstrained portfolio has considerably more flexibility. Since the unconstrained fund started in 2008, it has trailed the PIMCO flagship. But in the past three months, Pimco Unconstrained has gained an edge. Supporters of the unconstrained fund argue that it should eventually outperform. "Over a full market cycle, the flexibility of the unconstrained fund will prove to be an advantage," says Chris Sawyer, a research analyst for Litman Gregory. Sawyer says the unconstrained fund can adjust duration, a measure of a fund's sensitivity to changes in interest rates. The Barclays Capital U.S. Aggregate benchmark has a duration of 5.2 years. So if interest rates rise by a percentage point, the benchmark would lose about 5%. The Pimco flagship fund -- which has a duration of 4.7 years -- typically keeps its duration within two years of the figure for the benchmark. Pimco Unconstrained has a duration of 0.9. The short duration enables the fund to limit capital losses when rates rise. Scout Unconstrained has lowered its duration to -2. By running a negative duration, the fund can record gains when rates rise. Portfolio manager Mark Egan has achieved the negative duration by selling securities short, betting that rates will climb and prices will fall. In 2012, Scout returned 23.1%, ranking as the top-performing unconstrained fund. Egan achieved the big gains by holding bonds from leading banks, including Bank of America ( BAC) and Citigroup ( C). The bonds rallied sharply as investors gained confidence that banks were on the mend. These days, Egan is not finding many bargains. Instead, he is buying short-term bonds, which would suffer only limited losses if rates rise. "We are trying to protect the portfolio against the losses that would occur if interest rates rose only modestly," he says. Follow @StanLuxenbergThis article was written by an independent contributor, separate from TheStreet's regular news coverage.