Can you clarify what nystagmus really is?

Rapid eye(s) shaking. Nystagmus occurs frequently in those born with low vision in both eys, or conditions like albinism or aniridia. It may in part be a protective adaption to prevent too much light from hitting the retina. Sometimes it occurs with no eye problem as a wiring issue with the brain stem. Adults get short term nystagmus from intoxicants and some medications.
Dancing eyes. Nystagmus may be caused by eye muscle control problems or may be due to a congenital visual deficit. Evaluation by a pediatric ophthalmologist will help classify types and treatments for nystagmus.