Conduction is faster in the bypass tract than in the AV node (accounting for the delta wave) but the refractory period of the bypass tract is usually (but not always) longer than that of the AV node. For this reason, a critically timed early atrial premature beat will be able to propagate through the AV node to excite the ventricles in a normal fashion the bypass tract, but will not be able to enter the bypass tract because it will still be refractory, thereby creating unidirectional block.
The time required for the impulse to travel normally though the ventricles and to reach the distal or ventricular end of the bypass tract will have allowed it (the bypass tract) to recover its excitability and to conduct the impulse in a retrograde direction to re-excite the atria. This is referred to as an atrial “echo beat.” If, by this time, the normal AV nodal pathway has recovered its excitability, the impulse may then be able to re-enter it and re-excite the ventricles. This establishes a re-entry circuit in which the antegrade pathway is the AV node and the retrograde pathway is the bypass tract.