How do altcoins relate to Bitcoin price movements?

The correlation breaks for individual coins during their own catalysts (partnerships, listings, major news). But in general, a 5% BTC drop often causes 10-20% drops in smaller alts.

Bitcoin dominance drives the market. During BTC accumulation phases, BTC dominance rises and alts bleed in BTC terms. During bull runs, alts outperform. During crashes, alts fall harder (2-5x BTC percentage drop).