For date-specific messages: New messages scheduled in the past will not be sent. This means that you could create an email at 6:00 PM to send at 5:00 PM and the email will not send.
If you immediately change the send time to 6:01 PM, it will go out to anyone it has not already been sent to whose sequence was not canceled at 6:01 PM.
New contacts added to the sequence will only receive date messages scheduled after they've been added.
For step-specific messages: New messages added will be immediately sent to sequence contacts who haven't received the message yet, if the timing of that message is "in the past" (i.e. 1 day after form submission and it is currently 2 days after form submission). If the new message is timed in the future (i.e. 3 days after form submission), the contact will receive the message at that set time.
New contacts added to the sequence will only receive step messages where timing applies. This means if an email is set to go out 2 days before an event, and a contact is added 1 day before the event, they will not receive that message.