This awesome image effect is the one seen on Ludmilla Maury, this snippet was designed by Thom Aufresne. When you hover of the button which has a distorted effect on its border you will see the image mask stretch-slide from the left along with the hidden image and when you move the move away from the button the image stretch-slide out of view to the right.
If you are having trouble with the pen, try the archived copy on GitHub
See the Pen Masked image animation by Thomas Aufresne (@origine) on CodePen.0