Enigne braking means leaving the car in gear and allowing the friction of the engine to slow the car down. On an automatic, when you release the gas, the torque converter disengages the transmission from the engine and it offers little resistance to the drivetrain. When you engine brake, (like leaving the trans in DS or manual mode) the transmission and the rest of the drivetrain are still connected to the engine and puts a different stress on the motor and cylinders. This stress has been associated with better seating the rings of the cylinders.

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