There is the eccentric technique, there is eccentric weight training, and then there is Essentrics. All these can be confusing. As a start, let's understand the meaning of "eccentric."
If you bend your elbow, you are bringing your hand towards you (the center), and the motion is “concentric.” If you stretch your hand and your arm away from the body, the motion is “eccentric.” From these opposite concepts we have two fitness training techniques.
Eccentric muscle actions require 10% more strength than concentric muscle actions. Therefore eccentric training creates more power and strength.
Now, both eccentric weight training and Essentrics are based on the eccentric technique.
However, one uses external weights...
...while the other doesn't.
As an example, if you lift a dumbbell, you are using an external weight. If you lift just your arm, you are lifting the arm's weight.
What difference does that make?
When done with external weights such as dumbbells, eccentric weight training leads to micro tearing of muscle fibers, internal bleeding, and Delayed Onset Muscle Soreness (DOMS). You will feel stiff and sore after the training, and need to rest and heal for typically 48 hours before training again.
Essentrics, however, doesn’t use external weights but our own weight. When we tone our arms, for example, we lift and keep them extended and then pump with resistance. Since each arm weighs about the same as a watermelon, you are lifting a decent load! There is no comparable muscle damage as in adding dumbbells or other external weights. This is the why ballet dancers, who train eccentrically (without weights) and are confirmed to be the strongest of all athletes, can rehearse for hours a day and seven days a week. When you do Essentrics you won’t suffer from soreness or need any downtime. You can do Essentrics everyday!