In addition to taking screenshots, the Masterpiece app records time lapse videos of the drawing process.
The time lapse will record automatically as you're drawing. When you're finished with your drawing, press the checkmark at the bottom right of the screen. The app will take a final picture and then generate a video. Both your image and the time lapse video will be saved to your camera roll. You will also be given the option to share the image and the time lapse via email.
It's worth noting that the app will only generate a video when you've drawn for at least a couple minutes, otherwise, it won't have enough data to actually make a video.