This happens every year when the Russell is reconstituted.
The actual index funds can't buy the new stocks in the index until the actual date of the change. Used to be that would spike those stocks on that date.
The index fund managers did not like that as it compromised their returns. Now what happens is that OTHER funds or private equity groups figure out which stocks will be added, buy them in advance, and sell at a negotiated price to the index funds on Russell reconstitution day. All is negotiated in advance - something like WXYZ will join the index on June 30. Private equity fund has deal with index fund manager under which the equity firm will buy WXYZ sometime before 6/30 and agree to sell it to the index fund on 6/30 splitting the difference between what they paid and the closing price on 6/30. Works for both parties - index fund manager gets his position without spiking the stock, private equity guy gets a risk free profit on a 10 to 20 day hold.
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