I'll begin by saying that this isn't a great mass builder. Many would regard this as a finisher movement. It's great for those who are dealing with elbow pain or sore triceps. This is a very effective movement to shuttle a lot of blood into the muscle without a lot of tissue breakdown. Because of this, it can be used very nicely multiple times per week without any negative effect on strength. Actually, Louie used to have some of our lifters do 100 rep band push downs daily when their bench would become stale.
With this type of training it's very important to select movements that don't break muscle tissue down very much. The added benefit of this is you'll get a killer triceps pump. I know it doesn't mean shit, but it's awesome to leave the gym with jacked up triceps!
To set this movement up, attach two bands around the top of a power rack, chin bar or anything else that'll give you enough tension. Start on your knees. While keeping your elbows tucked, begin firing off alternate rep pressdowns. This is where the piston name comes from.
After reaching failure, stand up and keep going until you reach failure again. A few sets of these and you'll be jacked.
Training Mistakes:
Make sure to select a moderate band tension. This isn't a marathon so a good goal would be to get 20-30 reps per position.
I recommend you do this last in your training session. It's also great to do on your non-triceps training days if you're looking to add some volume to your triceps training or trying to overcome a growth or strength sticking point.