What does the center plate do for a discus and other stuff?

I am an intermediate thrower at the discus and wanted a few questions cleared before I buy my own discuses to throw. Firstly, what are ABS on discus and how does the center plate help with throwing a discus? What would I notice with a discus with and without the center plates? Also, what are high spin and low spin discuses and how are they different? So far I have only thrown rubber discuses and I am just starting to the point where I am able to throw metal ones. Thanks for any answers and I hope you guys can help me...=]