When you added a drop shadow to the text and animated the shadow in the previous section, you created an implied light source that moves over the text. For additional drama, you can animate the gradient behind the Text layer to create a separate moving light source that's hidden behind the text:
The Ramp (color gradient) effect depends on two colors: the Start Color (black, in this project) and the End Color (white, in this project). By animating only the X value for Stamp of Ramp from 240 to 440, you've animated the black end of the gradient from left to right. This technique creates a second implied light source that moves from right to left behind the text.