In November 2154, tampering by a group of Klingons caused a malfunction in the intermix chamber aboard the Enterprise NX-01. As a result, plasma pressure within the chamber began to rise, threatening to destroy the ship. In order to reduce the pressure, Enterprise had to maintain high warp until the damage could be repaired. (ENT: "Affliction")
In 2370, while posing as a thief named Galen, Captain Jean-Luc Picard manufactured an intermix chamber malfunction on board Arctus Baran's mercenary vessel, causing the ship to accelerate out of control and the plasma pressure to increase. (TNG: "Gambit, Part I")
In Mr. Scott's Guide to the Enterprise, the function of the intermix chamber of a refit Constitution-class is explained in detail. Both matter and antimatter flow simultaneously inside the intermix chamber. The chamber is divided into a vertical and a horizontal chamber that join together in main engineering. The horizontal chamber branches out to the nacelles and also powers all of the phaser banks. The vertical chamber is connected at the upper end to the impulse deflection crystal, that normally powers the impulse drive. At the bottom of the vertical chamber are the matter and antimatter tanks from which the reactants are drawn. There are several energy converters connected to the vertical chamber that provide plasma for the more basic ship functions not related to propulsion.