So often the tricep pushdown is done wrong. Some of the top things to focus on are:
1. Being sure to come all the way down. Notice at the very bottom of the exercise how I push the hands back just a little bit extra. You will immediately feel this.
2. Don't go to fast. It's easy to get caught up in the momentum of this exercise and allow the rope handle to control you. Instead of this, make sure your body does not move, only your arms.
3. The last thing you should be focusing on is the pause/squeeze at the very bottom of the exercise. Do this after every rep.