I just redeemed for an upgrade on some upcoming flights to qualify this. I am assuming this is double miles of what I already earned from AMEX during the qualifying time period.
For example, from July 1 to Sept. 30 - I earned 5,500 miles on qualifying expenditures on AMEX. $4500 normal expenditure, $500 DL ticket.
I have already earned 4500 miles and 1000 miles for these expenses, total 5500.
I qualified for the promotion. So when it posts I should earn an additional 4500 miles (double what I already earned) and 500 miles (triple what I already earned double on). So I will get an additional 5000 miles for this promotion?
In total I would have earned 4500 + 1000 + 4500 (double promo) + 500 (triple promo) = 10,500 (with the first 5500 posted normally and the additional 5000 posted 6 to 8 weeks after 9/30).