Why work with elastics?
Bodybuilding elastics, training bands, resistance bands... Not always easy to know where you are with all the different names for basically the same thing.
These elastic bands are used in two cases: to add resistance and thus increase the difficulty of a movement or conversely to assist a movement by countering your weight.
This material works with a variable increasing resistance. Translation: the more the elastics are taut, the higher the resistance. The resistance level is proportional to the movement. When unstretched (usually the starting point of your movement), the elastic is under minimum tension. When fully stretched (so at the end of your movement), the elastic is under maximum tension.
In short, as you pull or push and the elastic band is stretched, your muscles need to provide extra energy to carry out the movement.