The right eye is up and out while trying to look straight ahead, which reveals an unopposed inferior oblique. The paired muscle to the inferior oblique is the superior oblique which is not working, hence this is a right trochlear nerve palsy. When one holds one's head down and to the left, the right eye is naturally in that up and out position in order to look straight ahead, so there is no double vision until he attempts to look down and in with his right eye. In that position, the right eye's axis is at a right angle to the inferior rectus, so the only muscle that can pull the eye down is the superior oblique. Since it is not working, that's when the palsy and double vision become evident. See if you can answer the question on the first page of the diploplia module now: 

During the H-test, a patient has the most notable double vision while trying to look to the right and down. If this is due to an isolated nerve disorder, which of the following muscles is most likely not functioning normally? 

1. left lateral rectus

2. right medial rectus

3. right superior oblique

4. left superior oblique

5. right inferior oblique

6. left inferior oblique

Click "continue" for the answer and another H-test.