The triceps extension machine is an isolation exercise for the triceps. The triceps make up two thirds of your arm, so building these muscles is essential if you want big arms.
It is generally better to use free weights and cables rather than machines, Because the free weights require your body to use more stabilizing muscles to hold the weight in place. The machines usually leave out these muscles, providing a less functional movement that might cause some strength imbalances.
The machine is easier to use and safer for beginners, requiring less attention to form and posture. So if you're a novice who doesn't train often, the machine might be a better fit for you.
The machine also gives you more isolation of the triceps muscle, and support that hold your elbows in place. Making the tension stay on the triceps.
There are many different types of tricep extension machines, but all do the same movement, which is the triceps main function - to extend your arm.
Keep your upper arms in contact with the pad at all times throughout the movement. Otherwise, secondary muscles will take the tension off your triceps.
Adjust the seat in a way that your upper arms are fully on the pads. And you can sit with the back firmly against the seat pad.
Do the movement with a controlled manner, do not allow momentum to take over, an don't jerk the handles.