Relaxation won‘t come if you just say, “relax”. Relaxation comes only when you are aware of what tension is. You do an asana (posture); you tense and then you relax.
You move and bend and relax; Flexing the muscles then extending the muscles to relax.
The fight or flight energy which is built up in the muscles, which you could not diffuse because you cannot run away in your home or job, you can diffuse in yoga exercise. In yoga practice exercise is followed by relaxation: you contract, you relax; you contract, you relax; you contract, you relax.