In atrial fibrillation (A Fib) an increased irritability of all the cardiac cells in the atria exists. Because of this increased atrial irritability, many sites within the atria attempt to initiate electrical impulses. Because all of the impulses are competing against one another, proper conduction (depolarization) cannot be completed and thus a quivering movement (fibrillatory wave) occurs. After the re-polarization of the ventricles is complete the AV node is willing to accept another electrical impulse. The first available impulse received from the atria will then be conducted fully. This mechanism is responsible for the irregularity of the rhythm.
I like this. Great little animation.
Muslim’s Pilgrimage: Mecca 1953