This article sort of skipped over the actual "gamma" part. When the banks buy stocks to hedge their calls, they push the price up. Why does that create a feed

This article sort of skipped over the actual "gamma" part. When the banks buy stocks to hedge their calls, they push the price up. Why does that create a feedback loop? Because the price going up makes each call more risky, which means they need to buy more stock for each call on their books (including the ones they already hedged). That effect, the relationship between the price of the underlying and how much you hedge, is "gamma".