Mr. Jacobs is a Minneapolis-based financier who has made hundreds of millions of dollars buying shares of Conseco, another heavily shorted stock.
Crook said: "We have been quite heavily shorted over the last 12 months and those people have lost a lot of money".
Some of the more heavily shorted Chinese stocks listed on NASDAQ, an American exchange, are considered by pessimists to have poor-quality profits and to be hard to understand.
But Petrofac has put on 44p to £13.09p after the oil services group - whose shares have been heavily shorted on concerns about its move into a challenging offshore market - said it expected a stronger second half performance.
Companies whose stocks are heavily shorted often try to engineer "short squeezes," pushing their stocks higher so that short-sellers face margin calls and have to cover their positions.
Beware of stocks that are heavily shorted.
In fact, stocks with high short interest continue to underperform for two to three years after they're no longer heavily shorted.
Prices of stocks that were heavily shorted last year for tax purposes are in for a boost this month, maybe 15%, says Willens, as people buy replacement stock.
