Hi Marinas Edge,
That checkbox is used to control what happens to the scheduled emails already in place for current bookings. So when you add or update templates, it allows you to update all the current bookings in bulk instead of having to run around and manually touch each one.
If a booking has already sent a scheduled email for that type of template, it won't be sent again. To see what I mean, open a future booking, go to the Email tab and look at the bottom of the page. See the scheduled email there? You can add, update or delete the scheduled email at the booking level.
The reason you got 50 emails all of a sudden is because you added a booking that was 0 days after booked and you had 50 future bookings. When you saved the template, the system went and created a scheduled email for all of those future 50 bookings and since the scheduled sending time (0 days after booked) was past, they all went out.
Does this make sense?