Commentary for Avodah Zarah 145:4

According to R. Dimi, when each drop falls into the vat it is annulled because it is such a small amount. Once it is annulled, it becomes part of the undifferentiated wine in the vat and then the next drop is annulled. At a certain point the majority of wine was originally yayin nesekh and has, in a sense, been made kosher.