If you've got a few hours to kill:

http://spaceflight.nasa.gov/shuttle/...hutref/events/

Sounds like essentially what Dudley said:

To keep the dynamic pressure on the vehicle below a specified level, on
the order of 580 pounds per square foot (max q), the main engines are
throttled down at approximately 26 seconds and throttled back up at
approximately 60 seconds. This also reduces heating on the vehicle.