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FCP X only has basic masks for Ovals and Squares. Using CoreMelt's SliceX product you can create spline masks of any number of points and track them to movement using the academy award winning Mocha tracking engine. Isolate parts of the clip for color correction, vignettes, blurs, witness protection, adding depth of field, removing unwanted objects and many other uses.
This tutorial show you how you can use freeform shape masks with any built in effect or third party plugin by duplicating a layer and using the "Shape Mask Layer" plugin.