The gdx and gdxj rules are laid out in documents on this website. http://www.mvis-indices.com/
You have to make yourself a free account to log in. Basically if money pours into these indices, it will drive up the prices in the underlying equities also so the increased actual proportion of the companies required for them to hold is buffered. But if they get above about 6% of their portfolio is in one stock, they start off loading that one. And if they need to include more stocks in their index, then they can do that once every three months in a rebalance of the indices.
You'll get a much better idea if you read through the prospectus for the fund.
