Until now every calendar was checked on a timer. That is fine for a booking made an hour ahead and useless in the case that actually hurts: someone books you while you are still on the call with them, and the other calendar does not know for another minute. Keeper.sh now subscribes to the notifications Google Calendar and Microsoft Graph send when something changes, so the copy is made as the change happens rather than when the next check comes around.
Polling has not gone anywhere. It runs exactly as it did, and push arrives on top of it, which means a missed or expired notification costs you the old behaviour rather than a silent gap. Subscriptions are renewed before they lapse and their health is tracked, so a provider that stops delivering falls back to the timer instead of quietly going stale.
This covers Google and Outlook, because they are the providers that offer it. iCloud, Fastmail and other CalDAV calendars are still checked on a schedule — Apple publishes no way for a calendar tool to be told an event changed, so every product in this category polls them. Realtime is part of a paid plan, and an instance you run yourself gets it once you give it a public address the providers can reach.
Read more about what a paid plan adds.