1. Launch After Effects and create a new project.

2. Create a new composition.

3. Import any type of footage into the project and drag it to the timeline to create a layer, then select it on the timeline.

4. From the menu, choose Layer -> Mask -> New Mask.

5. Work your way down to the "Masks" property on your layer by clicking on the triangles. You should eventually reveal lines that let you control four mask properties: Mask shape, feather, opacity and expansion.

6. Hold down ALT, SHIFT and CONTROL, then click on the words "Mask Expansion" (the label for the property right next to the little stopwatch icon).

If you do this right, the label for the property changes from "Mask Expansion" to "Mask Embiggen"