This experiment uses the same technology as photography. For a few moments you can say hello to your shadow, before it slowly fades away.
The flash of light impinges on the screen and activates its atoms. The screen has been coated with a fluorescent substance. The electrons are given a higher energy and the colour of the screen lightens. However, nothing happens where there was something in the way, and a shadow is created. The electrons slowly calm down, emit their energy and return to their normal energy states. This energy leaves the wall in the form of light with a characteristic green colour.
When all electrons have returned to the ground state the complete wall has become dark, as it was in the beginning. This takes a certain time, and while it is happening the shadow remains as a frozen image.