Often I see individuals do tricep pressdowns and they move the bar in a very short movement only at the bottom. What is being missed is stressing the muscle over a full range of movement. Since gravity and in the case of the pulley and weight stack are moving vertically there is little to no resistance in the upper half of the movement. A way to remedy this is to do some push downs standing and some kneeling to hit the upper part of the range of motion. When your arm is perpendicular to the line of resistance it gets harder. Which is a good thing.