Shorts cant cover before volume come into the stock. Otherwise the price will increase to much and to fast. Thats the reason they dont cover. I hope we will increase in PPS slowely up to 5 before the good news.
Not an expert on shorting but my understanding is shorts / naked shorts only have to cover when NWBO is acquired and all open trades need to be closed out. However until then and in the meantime they are taking on high risk at this low PPS and paying a high borrowing rate, ie XX%. When news breaks out and a sustained upswing in price is clear, they will want to cover and reshort later or not.
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