The smaller hole is for the pinhole test. The pinhole test restricts light to the center of the patient's lens. This way, refractive errors due to a misshapen lens are eliminated and it is possible to determine whether the patient would benefit from corrective lenses.
The larger hole is for the ocular dominance test. The patient is instructed to look at an object with both eyes through the large hole. While holding the device steady, the patient is asked to close the left eye and then the right eye. If the left eye still can see the target with the right eye closed, the patient is left eye dominant. If the right eye can still see the target with the left eye closed, the patient is right eye dominant.
The Beren's Pinhole Test is very versatile. It can be used by hand or placed in a standard trial frame or flipper.