Crude oil is pacing for its worst weekly performance in the last three months as concerns about slowing global growth and a strengthening dollar have commodity traders hitting the sell button. And that has one group of traders very happy: short sellers.

"Open interest increased by 33,000 contacts on [Wednesday], so that's a significant addition of shorts," BNP Paribas' head of U.S. cash equity trading, Darren Wolfberg, said Thursday on CNBC's "Futures Now." Government data released Thursday showed U.S. supplies rose for the seventh straight week.

Wolfberg explained that when open interest rises on a day that an asset is down — crude oil fell 3 percent Wednesday — it means that traders are making new bets that the price will go lower. Conversely, he added that in early October, the increase in open interest was tied to a move higher in crude oil, reflecting more long positions.

"This is actually a good barometer to know the underlying flows of the marketplace," he said. When investors go short, they are betting a security or commodity will fall in value.

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Oil has been particularly volatile in 2015. The commodity has moved an average of 2 percent in either direction each day this year, according to data from Factset, making it one of the most volatile assets in the market. But for those searching for clues on how to trade its unpredictable swings, Wolfberg stressed the importance of "being nimble, identifying stop levels" and knowing where "to add positions and take profits." He is eyeing $40 as the next breaking point for oil.

"The momentum is clearly down and it looks like oil is heading to that $39 to $37 level that we saw a few months ago," added "Futures Now" trader Jim Iuorio. "There's very little that could make me be the first guy to stand in front of a room and say buy oil right now."

Crude oil closed Thursday down 3 percent, at its lowest level since Aug. 26.