aescripts

// Applies Echo effect to selected layers with settings that work like onion Skin from Adobe Flash.

var activeComp = app.project.activeItem;
if(activeComp){
  var selLayers = activeComp.selectedLayers;
  for(var i = 0; i< selLayers.length; i++){
    var echoEffect = selLayers[i]("Effects").addProperty("Echo");
    echoEffect(2).setValue(9); //Adjust EchoTime
    echoEffect(4).setValue(.5); //Adjust Decay
  }