Yoga Sutra 2.47 says that by relaxing EFFORT in yoga postures, the yogi merges with their focal point.
There are 2 points here worth considering.
1. Relaxation of strain/effort is an ability that comes with time and regular practice. There is not a complete lack of muscular tone, working with breath, moving deeper into the posture. Rather there can be a relinquishing of OVER-TRYING. I like to think of non-efforting as EASING, finding the space within the engagement, ACCEPTANCE of the pose as it is.
2. Through this relaxation, the YOGI can merge with the intended focus. What is our point of concentration? To where are we directing our moment-to-moment awareness? The more we seek out qualities of EASE, LIGHTNESS and GRATITUDE, the more likely it is that we'll be absorbed in those experiences when we release grasping. This state of being implies a CALM + SETTLED heart-mind, which is undisturbed by the whirlings of the external realm.