For this comparison post I decided to use pictures of myself as that seemed to me to be more fair than using pictures of anyone else! I'm riding a client's cute little warmblood gelding who I had ridden maybe 5 times before this event.
Look at both pictures carefully and see if you can tell the difference in the way this horse is going. Although they are taken in different phases of the trot stride, it's easy to tell which shows more real engagement. In the top picture, we are having a really good moment in our dressage test where I asked him to move forward into the bridle and he did. This is a fairly lazy gelding so usually when I would ask him to reach for the bit he would drop his back and give me a "head down" as an answer. But in this moment, his inside hind was reaching under his body, his back is up, and he is using his hind end well. I love this picture because you can even see his hindquarter muscles working!
In the lower picture, he is "sucked back" and doesn't have any drive from the hind end. In some horses, this looks quite different as they will put their head straight up and drop their back more visibly. This horse had learned to evade by curling his head toward his chest and just moving his feet faster. When I was riding him it was important for me to remember that engagement comes when the horse moves forward in a rhythm and uses his whole body to trot-not just his feet.