STOCK COMPENSATION - Excess tax benefits in cash flow statement

I have never fully understood the excess tax benefit in a firm's cash flow statement. it reduces cash from operations and is added to cash from financing. Both entries are shown. What is the definition of an excess tax benefit. Is it benefits above tax rate x value used to amortize (say option expenses) should the stock rise. i know there are all kinds of complications with additional paid in capital. looking for simplest explanation