Because of my somatic training my observation of the human body is constant. I often witness people struggling to move in forward motion. So over time I realized that the tailbone often "hangs back" or "tips up and out behind" (due to a weak core). So if you are walking forward but your tailbone is "hanging back" this is a confusing message to the body. The mind says lets go forward and the body says I am trying but the tailbone just keeps dragging behind.
At the tail of the spine is the coccyx which is the bony fused spine that tips forward or curves underneath us like a little hand scooping you forward. Go ahead and stand up and bring your tailbone through. Feel the the engagement of your lower core and the forward motion it creates. Now walk with the tailbone coming through towards the pubic bone. We are not clenching the buttocks or tightening. We are just allowing our tailbone to through move through the front of the body towards the pubic bone to create impulsion or forward motion. Allow your legs to lengthen their stride and release the front of the hips and legs. Feel the shifting of each stride ride up the spine into the shoulder and down the spine into the hip. Keep lengthening your stride by bringing your tailbone through - no need to clench the buttocks. Open at your solar plexus and clavicle and feel the forward motion. This is impulsion through the tailbone. Practice, practice, practice.