Tubby's post is basically correct except for the fact that shaft bowing has probably already occurred by the engine sitting without the shafts turning. Think of a dumbbell with heavy weights on both ends (fan & booster ---- LPT).
Although the engine dynamics are even more complex, here is a simple explanation of the IAE situation. Shortly after engine shutdown, heat begins to build up in the top portion of the engine due to the heat retained in the engine components and lack of venting for this trapped heat. This so-called "chimney effect" can deflect (bow) the low and high-pressure shafts due to the diametral temperature gradient. The maximum deflection generally occurs between 1 to 2 hours after engine shutdown. Therefore, if an aircraft's engines are restarted during that period, a bowed rotor start can occur that could create a rub condition along with rotor dynamic issues. Bowed rotor starts have been known to actually lockup the low-pressure shaft due to the LPT blade interference with the LPT shroud. By keeping the rotational speeds as low as possible for a period of time at idle speed, both the rotors and stationary components have a chance to stabilize which helps preserve clearances and therefore engine efficiency.
Hope this helps...