After the center of the feet, usually the first "easy" idea I teach to new students is paying attention to the body line. When the hand (or more precisely, the point of contact) is inside the body line, it is easier to absorb. Conversely, when the hand crosses outside of the body line, it is easier to project force. The body line is an important transition point which needs to be recognized to maintain unification with an opponent's force.
It's a simple concept that is easy to demonstrate. Just move the hand inside or outside the body line and try absorbing to pull or projecting to push against a partner's force. The importance of recognizing open vs closed becomes evident just from the touch feedback. Absorbing while open or projecting while closed only generates power from the arms and is harder. Projecting while open or absorbing when closed links up more joints in the body and generates more power with less effort.
Why this should be the case might not be immediately obvious. A quick thought experiment can explain the body line transition. Imagine the shoulder as a center of rotation for the arm in the horizontal plane. The point at which the arm is at its front-most position is directly perpendicular to the body (i.e. at the body line). If we borrow some math from the previous post on spheres of offense and defense, we can treat the body line direction as a diameter line of a circle. As the arm crosses inside or outside the body line, the diameter line is crossed. The forward-back motion vector of the arm (i.e. the tangent the arc) switches sign upon crossing the body line.
Alternatively, we can use a clock as our circular motion model. When the minute hand goes from 9 to 12, there is an upward movement component. Exactly at 12, there is no upward or backward movement component. After 12 is passed, the minute hand has a downward movement component. Once the hand crosses the transition point, there is a change from a forward to backward movement.
Of course, the human body does not move strictly according to rigid body mechanics, and movements usually involve several joints. The simple analysis is imperfect, but it serves as a rough approximation for understanding the mechanics.