Right; the fact that going higher in the SP will detract the shorters was multiple times mentioned but not really explained and this is because it is not exactly an exact science.
My view on this (and I think this is the main reason) is the high percentage of gain/loss.
Please take a look on the list of "Gainers & Losers" at the beginning of each HC page and you'll see in 99% of the cases just "penny stocks"; this is because of the volatility.
In case of the "Gainers" it is a huge profit but encounter an extremely high risk; in case of the "Losers" it is the best opportunity for shorters with the most profitable ground for practicing their games.
Ditto
