an WPW the impulse generated in the SA node pases directly to the from Right atrium to the Left atrium (because you need both atriums to contract right? something must pace the left guy also). So SA node sends impulses to the Left Atrium + to the AV nodes (which has refractory period that you mentioned). The accesory bundle, is in the left atrium. So every time when then left atrium get's an impulse it can propagate it to the ventricle, at it happens all the time, that's why you get that strange slope on ECG, the preexcitation upslope in the PR interval. AV node doesn't control the wpw story, if you give anything that slows down the AV node you are gone, wpw tachyarrhytmia is not the same principe like in supravent tachy, here the impulse reached the vent's without passing a single time the AV node, so anything slowing down AV, will send more impulses to the kent bundle and exacerbate your arrhytmia (Boards and beyond, I learned this from that resource)
