Question about the sympathetic/parasympathetic nervous systems.

I've been reading about this a lot, and some sources say an overactive sympathetic nervous system (fight or flight) can contribute to IBS symptoms, either constipation or diarrhea. These sources generally claim activating the parasympathetic nervous system (rest and digest) can help alleviate symptoms.

What I don't understand about this as someone who struggles with IBS-D is how activating the rest and digest state, which is supposed to get the gut moving again, would help with my frequency, urgency, and diarrhea. Wouldn't that just make things worse?