The trigger is both the timing and the opener. A new Head of Sales, a funding round, a job post for an SDR, a switch to a new CRM: each one is a moment when the account is reconsidering how it works, and a reason for your email to exist.
The rule is that the trigger has to connect to what you do. "Congrats on the round" followed by a generic pitch is a trigger wasted. "You just posted for two SDRs; here is how teams your size usually equip them" is a trigger used.