SpreadFade is named after the fading squares that hide the letters. The fading spreads through the area of the CAPTCHA.
This CAPTCHA defines two layers. The back layer has the text and the front layer has the squares. The squares are visible by default and fade to transparent to let the visitor see the letters. The fading spreads from the edges. The shape of the fading can be selected. A noise can be added to the spreading. This makes the edge of the spreading wider and not so accurate.
The weapon of this CAPTCHA: The method is to hide the information. There is no frame in the animation where all letters are visible at once.