A halving occurs when the permitted block subsidy shifts to the next issuance era. Because block times vary, its calendar date is estimated; the triggering block height is exact.
No organization activates a halving. Nodes apply the subsidy rule when validating the relevant height. The first halving in 2012 reduced the subsidy from 50 to 25 BTC; later eras continued the same integer division.
A halving reduces new flow, not existing supply, and does not mechanically guarantee a price increase. Its economic effect interacts with anticipated demand, miner costs, fee revenue, inventory and expectations already reflected in markets.
Triggered by block height
Every 210,000 blocks
Reduces new issuance, not existing supply
Does not guarantee market returns