The document discusses the Jackson Cross Cylinder (JCC) test, which is used during refraction to detect and refine astigmatism. The JCC is a combination of two cylinders of equal strength but opposite signs, placed at right angles to each other. During the test, the JCC is held in different positions before the eye to see if there is a change in visual acuity. If a position is clearer, it indicates the axis of astigmatism. The test is then used to refine the axis and power of any astigmatic correction.