Rotorbow is when the engines cools down (engine shutdown between 20 minutes and 5 hours/depends where you are) and there are different temperature gradients with the engine. In other words, the temperature at the bottom, middle, and top of the engine are different. These temperature differences will bend hp shaft which will cause an imbalance; causes the airframe to vibrate violently. You will also notice your engine vibes will be high as well. The procedure that I'm aware of is you allow HP rpm to max (approx. 28%) out plus 30 seconds and then you continue with your normal start.
Hope that clears a little of the confusion.
