Water gushing or backing up out of the bathtub overflow drain while you are running the shower indicates a severe blockage in the P-trap or main drain line that prevents wastewater from escaping down the vertical pipe. To stop the gushing, turn off the water immediately. Use a cup plunger over the main drain while sealing the overflow plate with a wet rag to force the hair clog through the P-trap. Verification: Run warm water at full blast to confirm normal drainage without backflow.
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