Sam Harris
Can you completely relax, even if you feel tension in your body, or restlessness?
This is one of my current practice elements: completely relaxing the body while staying fully concentrated on my meditation object.
I scan the body for each remaining pocket of tension as I focus with full force.
Relaxing is not slumping
However: by "completely relaxing" I do not mean "slumping". Instead, I mean relaxing as much as possible while using the classical 7-point posture. That posture requires an equal distribution of a basic tension throughout be body.
Thus, what I mean is superfluous tension, such as drawing up the shoulders or facial tensions around the mouth or the eyes.
Tension and attention share a word root
Usually, when one focuses, or puts intensive attention on something, one feels some kind of muscular contraction: it can be around the eyes, the shoulders, the upper arms - whatever.
One will not be surprised, then, that "attention" and "tension" share a word root, namely the proto-Indian-European "ten" (to stretch), which is also the basis for thin (in German "dünn") and many other words.
Benefits of letting go of tension while paying attention
Feeling: It feels good: Releasing the physical tension while paying attention feels wonderful.
Health: it's good for the nervous system and the heart by, for example, reducing the muscle tension resulting from chronic stress. Of course, its even more healthy when one carries it as habit into daily life.
The Self: It reduces the felt sense of Self that is making an effort of getting something, or acquiring something
Receptivity: it increases the ability to just "receive", instead of making something happen (this point is somewhat related to the sense of Self)
Scope of attention: the relaxed mode of paying attention allows a simultaneous strengthening of the awareness-mode. The faculties of focused attention and peripheral awareness work together, as explained by Culadasa in "The Mind Illuminated".
Resources
Yates (Culadasa), J., & Immergut, M. (2017). The Mind Illuminated: A Complete Meditation Guide Integrating Buddhist Wisdom and Brain Science for Greater Mindfulness. Hay House Uk.