What is meant by a "pressure-balancing" tub and shower faucet?

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A pressure-balancing tub and shower faucet balances incoming hot and cold water pressure to compensate for fluctuations in water flow. Balancing the water flow keeps shower temperatures constant and comfortable, within ±3°F (1°C) of the original temperature, despite other demands such as flushing the toilet or starting the dishwasher. Moen's PosiTemp and Moentrol® tub and shower valves all feature pressure-balancing. Both valves require pressure on the hot and the cold side to operate.