Relaxing, allowing the connection that is natural, opens your body to feel the support of your earthly soul. Grounding then becomes an acknowledgement of support from the earth, and an equivalent acknowledgement of interaction between spirit and form. While you can continue to frame this interaction in terms of a plum line, you may want to explore other options. Perhaps even letting the line expand in diameter to connect with more of your body. One image that I've found helpful is that of sitting on a large ball.
Female bodies tend to like to ground on a higher percentage of female energy than male bodies.
Psychically it looks to me like a large ball of energy supporting the tailbone. Male bodies will have some amount of female grounding, so this idea can be useful for men also. Visualize sitting, even bouncing, on a large exercise or medicine ball. The entire buttocks, lower torso, knees, feet and ankles are swaying and moving with the motion of the ball. Grounding occurs within the space inside of the entire ball, and wherever the physical body is moving.