If a Nazir eats less-than-a-K`Zayis of grape product he is Patur.
Now say a Nazir dipped a piece of bread in wine and ate it, where the wine alone did not amount to a K`Zayis but together with the bread it did.
If all the bread was soaked, since all the bread has a taste of wine, we apply the concept of Taam K`Ikar and consider the bread as though it were wine.
If only some of the bread was soaked in wine, then the portion that does not have the taste of wine could not be considered as wine through Taam K`Ikar. However the concept of Heter Metztareif L`Issur can still be applied, we can add the permitted bread to the forbidden wine and he would be liable as though he had eaten a K`Zayis of wine.