Anatomy lesseon you asked for, anatomy lesson you'll get :O)
-the psoas muscle is a part of the hip flexor (the muscle that among other things lifts your leg, iliopsoas)
-pelvic floor muscles have various functions, such as
*holding up the organs of the abdominal cavity
*in coordination with the upper diaphragm regulate the intra-abdominal pressure.
Here comes into play also the (transversus abdominis) deep abdominal muscle, which is triggered via reflex upon activation of the pelvic floor (BTW p.f. can essentially be regarded as one, my guess is that the tailbone muscle you've found is the anal sphincter)
Next step (of importance to intra abdominal pressure and spinal stability) is tension (from contraction of transversus abdominis)
in the thoracolumbal fascia, which in turn (via reflex again) activates the deep dorsal muscles, such as the multifidi (among others)
*the pelvic floor muscles can also help you in times of need, when you're not close to a bathroom
So you see, activating the pelvic floor muscles help you gain spinal stability via the deep muscular system, leaving the long, strong dynamic muscles relaxed and free to be utilised for movement.
This is imho more effective than pushing in your lower abdomen, because that cannot be done without activating more superficial parts of the abdominal muscles (rectus- and obliquus abdominis) and unnecessary tension in them will effect freedom of movement and breath.
If this post smells of besserwisser, it's just because I know so much
[This message has been edited by nenipp (edited 08-05-2004).]