This is one of the most difficult things to teach animation students. For some reason, they seem to either ignore or completely eliminate this principle. It's my opinion that this is due largely to lazyness. A recovery/reaction requires between 5 - 11 extra drawings on the end of an action to allow the character to settle into a relaxed position.
Not every action requires a reaction. If the action is very fast and the character is tense, or the intent is to show strength going into a hold position, then a reaction would be counter productive to the effect required. It is also true that some styles of animation do not require this principle.
However, softer actions often do require a reaction and final recovery. By adding these in, you will create more of a sense of gravity acting upon the character.