It depends on the type and liquidity with-in the the fund. There are two basic types of ETFs ... in-kind species and swap. Many fixed income ETFs may not have adequate liquidity, however, option trading (although risky) allows methods of betting against most ETFs. Research is a key. The following link provides a starting point ....
http://www.nasdaq.com/investing/etfs/trading-options-trading-ETFs-Part1.aspx