Eligibility usually requires an exchange listing, a minimum market value and float, sufficient liquidity, domicile, and often positive earnings over a trailing period. Announcements come days before the effective date, and index funds mostly trade at the closing-auction on the effective date because that is the price the benchmark uses.
The result is a predictable pattern: a jump on announcement, drift into the event, then frequent give-back afterwards as the forced buying ends. The size of the move scales with how much index money must buy relative to the stock's daily volume.
Example: funds tracking the index hold $9T and the stock's weight will be 0.06%, so $5.4B must be bought. With $400M of average daily volume, that is 13.5 days of turnover arriving in one auction.
Related: index-effect, index-rebalance, free-float-weighting, closing-auction, index-provider