To create motion in After Effects, you need to activate the layer's properties linked to elements added to the Timeline. However, while you may initially think you can only animate imported illustrations or shapes created with the software’s tools, it is handy to know that After Effects can also give life to your masks.

This trick is useful when you want to enclose a portion of your videos inside a particular figure for just a moment and then include more parts as the shape opens up. Follow these steps to expand and add motion to a mask in After Effects.

• Create two solid backgrounds of different colors by clicking Ctrl + Y or Cmd + Y if you are working on macOS.

• If one solid background completely overlaps the other, change the order in which their layers appear on the Timeline so that the second solid you created is below the first one.

• Move both backdrops so that each one occupies one-half of the composition screen.

Expanding a Mask in After Effects

Now that you have the canvas divided between the two backgrounds, draw a Mask on the first solid you created: 

• Activate the Pen Tool in the toolbar or by clicking G, and start dotting until you create a custom shape. Don't panic if the rest of your background disappears, that's the goal.

• Go to the Timeline and open the properties of the layer linked to the mask you just drew. Click on the 'Mask' tab and then on 'Mask 1'.

• Place the Time Indicator at the beginning of the Timeline and turn on the stopwatch next to the Mask Path option.

• Move the Time Indicator again a few seconds forward on the Timeline, and, in the same Mask Path option, click on the Shape tab highlighted in blue.
  

• When the Mask Shape window opens, increase the value showing in the Right option by clicking and holding, and move the cursor to the right. Then hit Ok.

If you followed the steps correctly, you should see two keyframes on the Timeline and a change of size in your mask as well.

To see the motion you have just created, place the Time Indicator at the beginning of your Timeline again and hit the spacebar to see how your mask progressively expands.

Adding the finishing touches while animating your mask's expansion

It's ok if you see that your mask has two colors, because you created two different solids at the beginning, remember? For this reason, your mask will take into account the first background until its endpoint, and then it will grab part of the second one.

To open even more the masking, repeat the last four steps but in the Mask Expansion property, and increase its values when you are about to generate the second keyframe on the Timeline. Then not only will your mask stretch horizontally, but it will scale up to take more parts of your video.

Use this animation trick when you want to select a part of your footage and gradually open its visibility.