In "My Last Duchess," choices about what to communicate and what to withhold are the means by which power is wielded. The Duke sees communicating openly and honestly with someone about the problems you have with their behavior as impossible because it would compromise his authority. It’s also possible to hint at his power by intentionally letting stories of the past exploits slip to a new listener. However, because language is full of subtlety, the Duke might accidentally communicate more than he meant to about his own psychosis.
The Duke is only able to describe his maniacal feelings to someone who is not the object of those feelings.