The Echo effect in After Effects combines frames from different times in a layer to create a visual trail, useful for animating motion or creating smearing or streaking effects.
First, note that your layer must have motion within. For example, a video clip of a person breakdancing would work. If you layer is a still object that you move by changing keyframes, the Echo effect will not have any effect on this layer unless you precompose it.
Once you apply the Echo effect, there are several effect settings to note. Echo time is the time between echoes. Note that is you have a negative value, echoes will be created from previous frames. But, a positive value will create echoes from future frames.
Then, choose the number of echoes, your starting intensity (or opacity) and the decay (falloff based on the ratio of an echo’s opacity to the preceding echo’s opacity).
Finally, the echo operator combines your echoes. The default of Add adds the pixel values of the echo together, which can create a high intensity white look. Other settings include maximum pixel value, minimum pixel value, screen (a less intense version of add), composite in back (current frame image appears in back), composite in front (current frame image appears in front) and blend (which averages the echoes).
Check out this short video tutorial to see the echo effect in action:
