AN-124 question.

I was up at EMA tonight and was fortunate enough to get there in time to see an AN-124 depart, as usual once lined up on 09 the engines were run at two different power settings for several minutes before commencing its take-off run. Why do these heavies have to do this? I’ve never found a definitive answer so I thought I’d ask you learned folk! 🙂

-Dazza