Told by an American who works for the FAA. 

He was at an air show and allowed to sit on the flight deck of one those giant 
Russian Anatov transporters during take off. The plane taxied to the end of the
runway and then sat there for 5 minutes with brakes on but with engines reving 
at full power. After take off he asked the pilot why they did this. The pilot 
replied "Pre-flight safety check. The manual says if the engines don't blow up 
after 5 mins at full power, then it's safe to take off."