Wheat Dextrin is the main ingredient in Benefiber - but yeah, they work the same way... you're right about the absorbing water functions... the benefit being that a little would help in creating peristalsis (bowel contractions) and softens up any compacted stool matter - it helps in diarrhea too where it will help form more solid-like waste, by removing the excess water in watery, non-bloody diarrhea...
taken from benefiber's site :
Wheat Dextrin is a natural soluble fiber. Wheat Dextrin is considered "Gluten-Free" which means it contains only a trace amount of gluten ( i.e., less than 10 ppm gluten).
That magnesium citrate is still kicking in a little bit- I had to come home hehe...

anyways good review because I have an exam related to all this stuff next week! We actually have a few constipation and diarrhea cases for pediatrics/adults... it's for a medical licensing exam where we get 12 standardized patients - presenting with common complaints like - chest pain, shortness of breath, bowel/urinary issues.... we have do a history/ focused physical and counsel them all in 15 mins and write a patient note in 10 minutes...