The fast pathway has a long refractory period whereas the slow pathway has a short refractory period. If a premature atrial beat occurs early enough to be in the refractory period of the fast pathway, conduction will not occur across this pathway, but conduction can still be possible by way of the slow pathway to reach the AV node. This begins a reentry tachycardia.
Depolarization / Repolarization / Absolute Refractory Period / Relative Refractory Period
I tell my students that in AVD the P waves simply never get the chance to conduct. They are always immediately in front, buried inside, or behind the QRS or somewhere in the ST segment or T wave (refractory period). I tell them that in order to make the call of complete block, you must find P waves in places where they have a legitimate opportunity to conduct - but they still fail to.