There are two types of pressure plates, a spring-type and a diaphragm-type, the diaphram type of pressure plate is the most popular type in cars today. The pressure plate consists of a clutch plate, springs, clutch cover and release fingers. The pressure plate is the unit the bolts directly to the flywheel. The pressure plates purpose is to apply pressure to the clutch disc allowing the drivetrain to slowly engage and provide torque thru the transmission to the wheels.
One of the most common symptoms that indicates a bad clutch is clutch chatter, this is felt when releasing the clutch pedal and feeling a jerking motion. This can also be an indication of oil on the clutch. Pressure plates do fail, and will in many cases fail with little or no warning. This is due to the mechanical portion of the plate breaking, or the fingers that release the diaphran to break.
There are a number of manufacturers that produce a performance pressure plate designed to produce much more clamp load with little or no increased petal effort. The use of a performance type of pressure plate varies and is used in most cases for vehicles that have been modified and produce more horsepower, or for trucks that do a lot of towing.