Iroquois MM, Chap 39, Page 1, Para 3 CAUTION
"The main rotor should not be turned backwards as damage to the main generator brushes may occur."
Interesting to note that this Caution is not in the Ground Handling section of the same manual. I don't have access to a -6, or POH.
Good to know my memory hasn't quite failed yet.
I can't find any other limitations.
As mentioned by others (thanks) this is only relevant for aircraft where the generator is connected to the txmn. Due to this feature, you can still have full electrical power when in engine off auto (landing light for night autos....don't even want to think about that!)