Information obtained from a thread courtesy of squeezemetrics via Twitter
You know how old-school traders talk about using the limit order book to know where supply and demand are? Basically what happened is that the limit order book got abstracted into options, and the actual buying and selling is now done by option dealers, on behalf of customers.
So what happens is, I sell a call because I want to collect some premium and give up some upside. This is largely equivalent to submitting a bunch of progressively larger limit sell orders above the market, except that those orders cannot be seen.
So what happens is that the option dealer takes the other side of those would-be limit orders by shorting the underlying and committing to, in this case, a schedule of buying the underlying when it goes down, and selling more when it goes up.
This means that there are, through the agency of the option dealer, a bunch of what are effectively limit orders sitting above and below the market, all of which will push against any move in the underlying itself. The more of these there are, the more stifling it gets.
So when GEX is really high, that’s when it’s super-stifling. BUT WAIT THERE’S MORE. As these call options lose value, the dealer slowly buys back some of the underlying that they shorted before, slowly supporting the market, ceteris paribus. Low volatility, upside drift.
(Hopefully it’s obvious why knowing where supply and demand will be is useful.)