In order to answer this question it is easiest to look at an example. Below you can see a graph which displays horizontal eye movements for a five second period. On the graph is a red and blue line which tells the examiner which eye is being recorded: red = right eye and blue = left eye. You can see on the graph that there appears to be some nystagmus (involuntary eye movement) present. But which direction is it beating?
Well in order to answer this, it is firstly important to know that jerk nystagmus (as displayed above) comprises of two parts: a slow phase which causes the eyes to drift in a certain direction and a fast phase which bring the eyes back to primary position (central position). It is convention to refer to nystagmus by describing the direction of the fast phase. In the example we have we can now label the slow phase and the fast based on the duration of the eye movement in each direction. We can see the eye drifts slowly to the right (upwards on the graph) for a duration of ≈ 1 second and then quickly moves back to centre by moving to the left (downwards on the graph). Therefore by using the naming convention described above we can identify that in this example the fast phase is moving the eye in a leftwards (downwards direction on the graph) and thus we call this left beating nystagmus. In VNG we typically measure in two channels so we can record left beating nystagmus, right beating nystagmus, up beating nystagmus, and down beating nystagmus for each eye.