In this tutorial you will learn how to use layer mask to create a show-a-part effect with detail explanations, easy to follow!

Step 1: First look for a picture you want to make effect with or you can use this one:

Press Ctrl-J to open Document Properties box and set the dimensions according to your image size

Then place your image onto your document, use Transform tool to make it full size if necessary

Step 2: Create a new layer, named it Mask and use the Oval Tool to draw a circle on place it at the left side of the image

Step 3:  Select frame 50, press F6 to insert a new keyframe and move your circle to the right

 Step 4: Right click on frame 1 and select Copy Frames

Click on frame 100, right click and select Paste Frames

Step 5: Right click on any frame between frame 0 and 50 and select Create Motion Tween

Repeat this step with any frame between frame 50 and 100

Step 6: Right click on Mask layer and select Mask to transfer this layer to a mask layer

Now press Ctrl-Enter to see the result:

This is a simple effect but commonly used in some different manners. Try to make another that moves from top to bottom!