Why the 10 Minute window?
It’s to prevent the overflow invasion from carrying over into the next hour when another zone is going to be hit by an invasion.
Each invasion is 45 minutes. With a 10 minute window, the latest an overflow will have the invasion is 55 minutes past the start, giving them a bit of a buffer for it to end before another invasion kicks off.
Yep—just found out about this, went to Frost as directed, put in overflow and do access to the achievement—incredibly shortsighted design decision. 2nd time in and unable to participate.
From my understanding, if you are somehow able to get into the overflows that existed during the 10-minute window, you can participate. Could be wrong about that.