The flex plate attaches to the engine crank shaft and serves two purposes.   It has gear teeth around the outer edge and when the starter is engaged the gear on the starter interlocks the gear wheel or flex plat.   This action cranks the engine allowing it to start. The flex plate is also connected to the automatic transmission torque converter.   The flex plate shown above has cracks around the mounting holes where it bolts to the engine crank shaft.   These cracks caused a knocking noise at idle and on acceleration.