During A-V dissociation there is no link between the atrial and ventricular activity. A good example is Complete A-V block in which the atria can have sinus rhythm and the ventricles a ventricular escape rhythm.
A-V dissociation, when observed, can be very helpful in the diagnosis of ventricular tachycardia. As a wide complex tachcyardia with AV dissociation is always a ventricular tachycardia. Sometimes, during ventricular tachycardia, the atria are activated retrogradely. Thus, A-V dissociation is not always present in ventricular tachycardia.