You would think they might have a method to the way they sell down the shares in those companies they're reducing in the ETF, but I don't see what that is yet. Even with the selling they've done so far in FF the percentage amount in the GDXJ has stayed close to 1% because of the reduction of other company shares.
I was told by Derek in IR that the stock performance didn't really affect what share amount was kept in the ETF, but that doesn't make much sense to me. I would think someone running GDXJ keeps up with individual performances for however long that's measured. 6-12 months anyway.