Chuck, don't laugh. You were supposed to turn the fan on the TFE 731 - Lear 30 series, Hawker 125's - by hand if you had to start them between twenty and forty minutes (or fifteen and thirty, I forget) after shutdown. The reason we were given was that because the dissimilar metals used in the construction of the engine had dissimilar cooldown rates, within those time limits something could be binding. The idea being that you wouldn't inadvertinetly gronch it with the starter, I suppose.