PSE's periodic review of which 30 companies make up the PSEi, normally taking effect in February and August, with off-cycle changes when a member delists or a new listing qualifies early. Because the basket is fixed at 30, nobody joins without somebody leaving, and funds tracking the index have to buy the entrant and sell the leaver around the effective date. That forced flow can move both stocks sharply without anything changing about either business.
See every term in the full PSE investing glossary.