1) Atrioventricular reentry tachycardia (WPW syndrome)
Electric signals issued by the sinus node are always transmitted to the ventricles via the atrioventricular node, which is located at the center of the heart, and cause ventricular contraction; there is no other pathway for the electric signals.
However, some people have another electric pathway (i.e., an accessory conduction pathway known as the kent bundle) that is located between an atrium and a ventricle near the atrioventricular node (normal conduction pathway).  This condition is known as  the WPW syndrome.
Unless a patient is experiencing tachycardia, electric signals are transmitted via  both pathways of the atrioventricular node: the (normal conduction pathway and the accessory conduction pathway. Therefore, patients with atrial fibrillation  have an electrocardiographic waveform different from that of healthy subjects even without rapid heart beating so we can diagnose WPW syndrome.)
If one has tachycardia, on the other hand, electric signals are transmitted to the atrium down through the atrioventricular node and up through the accessory conduction pathway to the atrium, forming an electric circuit that results in tachycardia. 

2) Atrioventricular nodal reentry tachycardia
In some patients, the  electric signals from the atrium  transmit slowly to the ventricles through the atrioventricular node   . In those patients who have two conduction pathways, of one way down and another way up,  an electric circuiting pathway forms  that results in tachycardia.