How does this work?

i can’t link it directly, but go to colorsmagazine.com, click on archive, then #55, the Birmingham issue… then click “welcome”

You will see a city photo divided into sections, when you rollover, the alpha changes, but then when you click on it, that part of the photo appears and stays visible at full alpha. How does the movie clip stay visible on the up state? after the first initial click…