It's the first time I'd heard it from a yoga teacher, too. She's a new teacher for me, and I mentioned that I had UC and was flaring to her right before class.
Essentially, I think she meant that if we tilt the pelvis forward, and use the lower abs a little, the internal organs will fit more naturally in the pelvis. If I think about standing straighter, my natural inclination is to pull my shoulders and butt back, with my stomach kind of sticking out in front. I think that's the opposite of what she was saying to do, at least for the butt and stomach. The butt should be more forward and in line with the rest of the body so the colon, etc., will sit naturally in the pelvic cavity. The abs gently hold the organs in so they will be better protected from gravity.
Does that make more sense? I think she used the term "bowl" as a description, not as a yoga term. She had us do a lot of lower back strenthening, as well, which helps with the abs.
I believe she is an ivengar teacher, and my usual teacher does hatha yoga. If I get any more tips I'll try to pass them on.