The second method is carried out by having the patient lie on his/her left side with the knees extended.

The examiner holds the patient's right thigh and passively extends the hip.

The pain results because the psoas borders the peritoneal cavity, so stretching (by hyperextension at the hip) or contraction (by flexion of the hip) of the muscles causes friction against nearby inflamed tissues.

In particular, the right iliopsoas muscle lies under the appendix when the patient is supine, so a positive psoas sign on the right may suggest appendicitis.

It may also be positive with other sources of retroperitoneal irritation, e.g. as caused by hemorrhage of an iliac vessel.