The feeling of the "tailbone" and dropping the qi involves relaxing the lower back and dropping the "tail".
Sifu emphasizes using the "energetic tail" as a counter-balance to many movements. However, when one of the senior-most students of Sifu visited my home last friday, he emphasized the role of the tailbone/lower back and the relaxation of the kua.
After following his directives, I noticed that the tail when dropped properly, aids in the qi flow to the legs and feet and the returning flow back to the mingmen point.
Also, the raising and sinking of the forms affects the "energetic" feeling of the tail and standing "too straight" ends up pinching the flow in the tail. So, always there is a little "sunken" physical form (not straightened/fully standing tall). The knees are bent slightly to allow the energy to not pinch in the tail.