Mark Christiansen shares some of the ins and outs of his Top Five Features in Adobe After Effects CS5 at Peachpit. Here's a taste of the 6-page article:
'Even better, you can simply copy that shape from Mocha AE and paste it into After Effects as a mask. Read that again, because this may be the biggest feature in After Effects CS5 that almost no one has discovered: Mocha AE can track a shape around an object over time, and you can copy that shape and paste it into a layer's Mask Shape, with keyframe data, right in the Timeline.
I know what you're thinking: "Weren't you just telling me about this other great automated roto tool?" The point is that no tool does everything. Roto Brush data cannot be converted to Mask Shape keyframes, and masks are embedded into so many operations in After Effects and its plug-ins that you could find yourself using Mocha AE as much as Roto Brush to create roto masks.'