Download this pdf file which will have all the Basics & important points required for you to understand Nystagmus.
Also check this link for Nystagmus videos.
First learn the basics of what happens when a particular Semicircular canal is stimulated to better understand Nystagmus due to vestibular origin.So check this image below-
. Stimulation of a single canal produces slow-phase movements of the eyes in a plane parallel to one in which the canal lies. As shown by the equations at the bottom, purely vertical nystagmus can only be induced by simultaneous stimulation of the same canal on both sides. Purely torsional nystagmus can only be produced by stimulation of both vertical canals, but not the lateral canal, on one side. Thus, disease of the labyrinth seldom produces purely vertical or purely torsional nystagmus. Combined involvement of all three canals on one side causes a mixed horizontal-torsional nystagmus.