Its the magnesium that relaxes the duct. Calcium is used by muscles to contract; magnesium to release. You should be able to verify this quite easily in most medical journals. All muscles respond in this manner which is why ER procedere includes giving IV magnesium to heart attack victims to relaxe their valves.

Here's a thorough description of magnesium. Scroll down to the heading titled "Functions" if you don't want to read the whole article.

http://www.bodyandfitness.com/Information/Health/Research/magnesium...

Sorry I don't know how the ES relaxes the ducts, but I can give textimony to the fact that if your bowels are empty (by enema or colonic irrigation) prior to the flush, meaning cleaning the bowels some hours before the flush the epson salts will have no diarrhea effect, it will just relax the ducks.
I am aware of this because I am quite sensitive to anything having a laxative effect so on my first flush I took only one tea spoon of ES the morning after the flush and nothing happened re loosening the bowels, on the 2nd flush I took 2 tea spoon of ES at 6 and 8 PM and still no laxative effect was noticable but I had taken 2 enema during the day of the flush.
By the way Andreas Morrisz (?) insist on having a colonic in the 3 days after the flush , and for me he is quite right, because after the 2nd flush I started having cramps in the lower abdomen, they came and went and 2 weeks later were still there, so I had a colonic and the
practician who preformed the colonic said my colon was full of strange dark green lumps (!) and of gaz.
Since the colonic this unconfortable sensation is gone.
Tonight I do my third flush and, will use ES as Dr Clark recommends, after having done some (2 or 3) small enema during the afternoon, I am using ES without apprehension of diarrhea effect because I know there will be none.
Ghyslaine