It must happen. The interesting aspect of this process is that the muscle will not only quickly relax back to its pre-tensed state, but if it is allowed to rest, will become even more relaxed that it was. As this procedure of creating tension and then releasing it is applied to every major group of the body then all of these muscles will become more relaxed than when you started. The key to triggering the relaxation response in this manner is to take charge of the voluntary muscles by tensing them and forcing them into a state of relaxation. Once the muscles relax then the other components of the relaxation response will naturally follow.
Relaxed muscles require less oxygen so the breathing pattern slows and deepens.
The heart does not need to be beating so fast to carry oxygen out to tense muscles. Heart rate and blood pressure decline. Blood supply returns to the belly and indigestion resumes. The belly is calmed. Hands and feet warm up.