Hi Judith, Technically it's the base of your sacrum, not the actual coccyx that is dropping down and behind. The coccyx needs to keep it's curled shape!! It's the natural angle of the sacrum, that if extended, would land behind your feet. Images can help: the 3rd leg of a tripod, a dinosaur or kangaroo tail..... or go online or find anatomy books that show how angled the sacrum really is - an image that is reality! I could not use some of my favorite pictures in the filming because of copyright issues, but there are some lovely ones out there.
Also, release your gluts, release any locking in your knees (by having a gentle wiggle in your knees, not a muscular bending). And, dare I say this, release any gripping in your abs! And release any gripping in your pelvic floor muscles, especially the image of sitz bones coming together. Gripping anywhere, actually, will make it hard to feel.
Not sure if this helps, or answers what you need. Feel free to contact me directly if you have more specific questions.