The test is performed by passively extending the thigh of a patient with knees extended. In other words, the patient is positioned on his/her left side, and the right leg is extended behind the patient. If abdominal pain results, it is a positive psoas sign.
Because the right iliopsoas muscle lies under the appendix when the patient is supine, a "positive psoas sign" may suggest appendicitis.
It could also be done by flexing the hip to 90 degrees -- with flexed knee -- asking the patient to actively flex their thigh against resistance.