On cue.....

Anytime we are de-iced, we are required to perform a 'de-ice' run up on the engines at 30 minute intervals prior to flight. At low turbine speeds (idle), ice can form on the fan blades, and the run up you witnessed is essentially a preventative move that sheds any ice that has formed on those blades. If ice is present on the blades it can be shed fairly harmlessly at low run up speeds and prevents the further build of ice. If ice breaks off at high turbine speeds, the engine can suffer significant damage (picture big geese, engines, etc.).

Often times with the traffic behind us in the takeoff line, it's impossible to do that run up without possibly damaging the aircraft behind us with our jet blast. It is therefore done once we get out on the runway, and it's clear behind us. We'll tell the tower we need an extra minute for an engine run up. Run 'em up, clear 'em off, and off we go......

DRW