contractions

Ede_1943,

This is my understanding of the two muscle groups you are talking about.
The anal sphincter muscle controls the opening and closing of the anus itself. As you contract (tighten) that muscle with the Aneros inserted it squeezes against the tapered stem of your massager. This squeezing forces the stem to move so the narrowest part of the Aneros device becomes located at the tightest pressure point of the anal muscle. When you relax the anal sphincter the Aneros is free to move outward due to the contraction of the rectal muscles (expulsion of stool). Thus the anal muscle is the primary muscle controlling the in and out motion of the massager. The rectal muscles surrounding the rectum are not as strong as the anal sphincter muscles but nonetheless can effect the movement of your Aneros internally (the side to side action). I have not developed the ability to control them in the directed manner you have described, but have been trying to develop the antagonistic ability to contract both muscle groups simultaneously, as suggested by BF Mayfield, to induce involuntary contractions (precursors to the Super-O).
There is another group of muscles, the PC muscles (developed through Kegel exercises), which can also have a subtle but significant effect on your Aneros’ movement. These muscles should also be employed in combination with the others to aid movement and stimulation to your prostate.
I hope this makes sense to you, if other members have a differing understanding, hopefully they will add their input as well.

In addition to involuntary contractions of the muscles in the wall of the rectum, gently tensing the abdominal muscles, as one does for a bowel movement, will force the Aneros in an out direction. By balancing this with the sphincter muscles pulling it in, one can get subtle, sublime movements of the Aneros up and down the prostate. With a little practice, the pleasure can go on for hours.