If it is just a mild discomfort colic, maybe it helps them relax? Seriously! I have heard of mild cases where the horse is given one dose of a mild sedative and then they are fine.
My horse had a mild case once, I gave a dose of oil and what we call the towel treatment: Roll up a towel length wise, have someone stand on either side of horse, have towel under horse (like a girth) between them, apply firm upward pressure starting at girth area and slowly moving back to abdomen. Soon enough, the horse was passing gas and feeling better.
Now, it was a mild enough case he prob would have been fine on his own, but I like to think it gave him some relief. Just like if you have mild cramps, sometimes just pressing on your abdomen gives you a bit of relief. Who knows?