An example is ISON. Yesterday it closed >90% of the daily high price on huge volume with low float. Therefore, a gap up was likely. Since it is hard to borrow, another pump up was likely. It double topped today around 1PM on monster volume. Stoc's hit ~98 on 5min chart. That was the safest time to short and if shares were available, adding to the initial short position would help as it weakened. But, be prepared to cover if it takes out the high. Its price move was based on PR and the news may generate future earnings; but new divisions are costly to start up and the gap should act like a magnate and fill. Hope this helps.
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