Marvell Technology (MRVL) has been confirmed for inclusion in the S&P 500, triggering an after-hours rally driven by anticipation of mandatory passive-fund buying. S&P 500 additions require index-tracking ETFs and mutual funds — which collectively manage trillions in assets — to purchase shares before the effective inclusion date, creating a mechanical, price-insensitive bid that has historically lifted stocks 3-8% from announcement to inclusion.

Marvell's underlying fundamentals give the stock a credible anchor beyond the index event: revenue of $8.2B grew 42.1% YoY (FY2026), gross margins sit at 51%, and diluted EPS came in at $3.07. The company is a key beneficiary of AI-driven custom silicon and data center networking demand, which has driven the top-line acceleration. These numbers matter because they reduce the risk that active sellers overwhelm the passive bid.

The classic index-inclusion trade is long from announcement to effective date, then fade on or just after inclusion day — passive buyers have already accumulated by then, and momentum sellers often emerge. The window here is typically 1-3 weeks depending on when the effective date falls. The tension is whether MRVL's strong fundamental story sustains buying pressure beyond the mechanical event, or whether the stock gives back the inclusion pop once the forced buying is exhausted.

Key variables to watch: the announced effective inclusion date, the size of the float relative to index weight (which determines how much buying is required), and any concurrent sell-side price-target updates that could extend or compress the move. Short interest covering into the inclusion could also amplify the initial pop.