Rebalances are usually quarterly and are announced in advance so funds can prepare. Weights move for mundane reasons: a share-buyback cut the share count, a lock-up-period expiry raised the float, a secondary-offering added shares, or a spin-off changed the structure.
The trading is concentrated in the closing-auction of the effective date, which is why those sessions carry several times normal volume and why the close can print away from the last trade.
Example: a constituent's free float rises from 62% to 78% after a lock-up expiry. Its index weight rises by about a quarter, and funds tracking $4T with a 0.20% weight must buy roughly $2.1B of stock.
Related: index-inclusion, free-float-weighting, closing-auction, index-reconstitution, index-effect