By Gary Gordon of etfexpert.comThe iShares DJ Transportation Average Fund ( IYT) gained 0.59% Tuesday. Yet the exchange-traded fund ended the day with $87 million less in assets under management, meaning it shed 17.5% of total assets in a single trading session. Now add share volume into the equation. Volume for the S&P 500 was average Tuesday. Yet the volume on IYT was 2.25 times greater than normal. Extraordinary trading volume and/or exceptional selling into strength (or conversely, buying into weakness) often signify a turning point. Indeed, the last time that IYT had this much trading volume was in the first week of February -- smack in the middle of the correction's bottom. >>Want More ETFs? Visit Our ETF Screener Page It appears most of the outflow came from block trading activity from institutional investors. And although it is difficult to assess the significance of advisers selling nearly one-fifth of IYT's total assets into the teeth of the cyclical bull, it's clear that some folks believe it is time to take profits.