Impulse propagation, reentry and possible fibrillation in the heart:
Click on "sinus rhythm" to see normal activity
Click on "reentry" and you will see the effect of an extrabeat at the "wrong" moment, generating a reentrant rhythm.
When the "Fire" button appears you will be able to generate an extrabeat yoursel, by clicking on "Fire!" or on a pixel of the image, and see what happens.