Right after the start
Zabbix agent get's the list of active checks and for each check collect data and set next check like now+delay
Zabbix agent2 just schedules next check as now+'conditionally random' time (that is always less than now+delay). This allows to make data collection more even

But in some cases we always want to get data first time right after agent starts. For example system.uptime - to get information about host restart and fire trigger on that immediately

It will be usefull to add Agent configuration option or keys parameter to force this check if agent just started

Also let's add more information about this in our documentation