Stock Watch: How to track what stocks are bought next on S&P 500 index?

Hello, I've been doing quite a bit of reading and something I found rather interesting: Legal way to game the Market

I'm wondering, how exactly I would go about tracking what stocks are going to be bought next by S&P 500 indexes? As in, where can I see what they plan on buying next?

Any help would be much appreciated. Thank you.