As this is product related could you move this to the Product discussion board.
With Head Start selected, the program will begin processing approximately 12 hours prior to the scheduled time. Once processing starts, the program is locked.
As the program is now locked any changes affecting the people who have been included in the send will not reach them. Its the state that a person was in when the Head Start program began running that is used for that send.
There is a caution in the documentation: Head Start for Email Programs - Marketo Docs - Product Docs
Anyone from your audience who unsubscribes after program lock will still receive the email. We recommend adjusting your unsubscribe notification to reflect that unsubscribes may take 1-2 business days to process.
This was really helpful - thanks!