Metamucil - Yes, it actually works! But why/how?

I've had pan UC for 8 years and aside from a 2 month respite, I have never been in remission without using steroids. The main symptom, of course, is diarrhea and needing to go to the bathroom anywhere from 4-8 times a day. These days I am on Entocort, which is a steroid, but less insane than prednisone; it only works in the upper colon. It didn't end my diarrhea, though it did cut my number of trips in half.

Anyway, that's the context. I'd read some reports here from 1-3 years ago of people with UC trying Metamucil and having good results. I just want to reinforce this impression based on my own experience - I have been taking Metamucil wafers (2-3 a day) for the past 4 days and the results are impressive. Stool is bulked up, which means the discomfort and energy expended in actually going to the bathroom is massively better than it used to be. And I only have to go 2 or 3 times a day, which for me is miraculous because even with Entocort, on a regular day, I could go 3-4 times in the morning and then 2 more times later in the day.

So these results bring me to my main question: how does this thing actually work? I've read about psyllium husks and how they interact with water, but that didn't help me connect the dots. For one thing, I thought fiber aggravates the gut if it is already ulcerative? And for another thing, UC has always been about urgency caused by ulcers, not some need to expel small bits of waste simply because they are there, right?

At any rate, I can't recommend this thing enough. It looks like the Metamucil powder is all "sugar free" (At least at my CVS), and that has Aspartame, which makes me sick, so I avoid that and stick with the wafers.

(Other info: In addition to Entocort, I take Culturelle, Azasan, and Apriso; stopped VSL 3 1 month ago).