Power Display is a special type of Power Play that deals with excess of force when dealing with mundane tasks. If a door is locked, most role playing games call for lock picking to open it. In Nobilis, a single miracle of Aspect can allow a player to shatter the door with a single fist.
In most games, Power Play is considered a lack of Role Playing spirit and is frowned at. A game in which a character can defeat effortlessly every challenge because the player is very skilled in negotiation or just has the power to do so becomes boring and loses all the fun.
In Nobilis on the other hand, since Nobles already have enhanced powers by default, Power Play is encouraged as long as it keeps the game fun and enjoyable. Harder challenges can be made that involve the intelligence and wisdom of players instead of pure Attribute numbers. It's the HG's job to help the player to figure better actions to take.