Colonic Hydrotherapy is far from bullshit - what happens is that the colon will accumulate layers of black, rubbery goo ('mucoid plaque'). This is what you will certainly dislodge with proper cleansing. If you are feeling brave, google an image search.
The small intestine and colon is no unimportant part of the body - it is the route for ingestion of nutrients and expulsion of toxins. So when it's covered in goo it will work unefficiently.
Colonic Hydrotherapy just deals with the colon. To do the small intestine as well you need to take herbs and psyllium husks. If you do that, prepare for some very interesting bathroom visits