Your tone of voice should be empathetic, appreciative and understanding. Create an environment of safe negotiation between ego states. Allow the various parts to all feel comfortable with the outcome of the therapy and communication. There is no question as to the value of abreaction or catharsis when working with clients. When the client is in trance, you can allow an ego state to abreact, or, the whole self as seems appropriate.
Experience is the best guide and defining rules in a limited article is difficult and un-wise.
The client should be allowed to get ALL of his emotional repression vented. The client must learn that the victimization he experiences was not of his own doing. He was a victim, plain and simple. As the client re-experiences these traumatic events he begins to release the negative emotions. As the events are re-examined a desensitization to the event and the experience begins to occur draining the negative emotional energy from the ISE or engram.