Emotions developed as a reaction are stored in the mind permanently. These reactions are accumulated in the mind and create mental strain. This is more dangerous as it becomes a part of mind. Removing this strain is extremely difficult. There are few advance techniques, which help reduce this type of strain. Relaxation is one of those techniques.
Thinking process of the mind also creates strain on the mind and to reduce this strain, we have to reduce the thinking process itself.

Doing something is Strain.

Doing nothing is Relaxation.

Actually doing something to learn the technique of
"Doing Nothing"
is Relaxation technique.