Sacral inhibition would decrease gi motility by decreasing hyperparasympathetics to normal levels - so this would help with diarrhea by slowing down gi tract

Sacral rocking would increase parasympathetics which would increase GI motility - help tx constipation by allowing the movement of gastric contents now that parasympathetics are increased.

"The indications for sacral ART, or sacral rocking, include somatic dysfunction of the sacrum. Sacral ART is one of many techniques that can help to treat somatic dysfunction of the sacrum.[9] Sacral rocking is often a preferred modality when patients present with pelvic symptoms relating to the visceral organs, such as constipation and dysmenorrhea.[10] Sacral rocking requires minimal patient participation; however, the patient must be able to lie prone for several minutes." - from link below

Sources = class lectures and https://www.ncbi.nlm.nih.gov/books/NBK560529/#:~:text=Sacral%20ART%20is%20one%20of,such%20as%20constipation%20and%20dysmenorrhea.