Concentric and eccentric are two very important factors in weight lifting. For the last two to three years I have been really working with these movements. Concentric builds for size; eccentric builds for strength. If you are doing a curl with a dumbbell or a barbell, the movement up toward your chin is concentric. The movement from your chin toward your knees is eccentric.
I have really been working on the eccentric movement with all of my weight lifting exercises. When I say I am working on the eccentric motion, that does not mean I am going stupid slow; it means my motion is very controlled. Again, I do not mean 5 seconds or more; I do not believe in that slow of a movement. I believe in a very controlled eccentric movement, so that at any time you are able to adjust your speed. Eccentric motion has helped my greatly in always being able to achieve my increases.
Concentric movement is very important as well. The concentric speed should also be a controlled movement, but not as slow as the eccentric movement. The speed of your concentric movement should always be the same as if you were moving your maximum weight. I believe your warm-up weight in bench should move at the same speed as your heavy weight in bench. I think concentric and eccentric movement plays a very big part in weight lifting.