The Psoas sign is positive in retrocecal appendicitis because the inflamed appendix lies in close proximity to the psoas muscle. Stretching or contracting this muscle during hip extension causes irritation and pain, indicating involvement of the retroperitoneal structures. This sign helps differentiate retrocecal appendicitis from other positional variations, such as pelvic appendicitis, where different signs like the obturator sign may be more relevant. Recognizing this aids in precise clinical localization and surgical planning.

Reference: Sabiston Textbook of Surgery, 21st Edition.