Sub-acute intestinal obstruction is characterized by a partial blockage of the bowel, leading to a gradual accumulation of intestinal contents and gas. This results in progressive abdominal distension, a key clinical sign. The distension occurs due to impaired transit and buildup proximal to the obstruction site. Unlike complete obstruction, symptoms develop more slowly, allowing noticeable gradual abdominal swelling. Early identification of distension helps in timely surgical intervention to prevent complications like bowel ischemia or perforation.

Reference: Bailey & Love's Short Practice of Surgery, 27th Edition.