When you train your triceps, you are trying to work the three heads of the same muscle, to get the horseshoe shape that defines the triceps. The tricep pressdown works the lateral head, which runs down the outside edge of your arm.
The most important parts of this exercise are keeping an upright posture, your arms pinned into your sides and your shoulders pulled back. Leaning over or hunching will cause you to use shoulders and upper traps to move the weight.
Take a look at our video tutorial below to see the fine form points of this exercise, and if you need something to print off and take to the gym, you can find our step-by-step exercise technique further down this page.