Activists must file a schedule-13d within five business days of crossing 5%, and that filing is itself the catalyst most traders react to. The letter or presentation that follows lays out the demands.
Outcomes vary. Some campaigns end in a settlement giving the activist board seats, some in a proxy-fight, some in a sale. A dual-class-shares structure usually makes the whole exercise futile, which is worth checking before trading the news.
Example: a 13D discloses a 7.4% stake and a demand to spin off a division. The stock rises 11% on the filing. Six months later a settlement gives the activist two board seats and the divestiture is announced.
Related: proxy-fight, spin-off, poison-pill