Confused on BTC dominance and alt season timing

Been trying to get a handle on the whole $BTC dominance chart. I get the general idea – dominance drops, alts pop. But when does it actually matter? I've seen periods where dominance is dropping but alts are still getting hammered because $BTC is taking a dump. So, for those of you who trade alt season, what's your primary trigger or confluence factor that signals it's actually time to rotate into alts, rather than just another dead cat bounce for them?