Snorting the same as rectal application (and IV) bypasses the first pass metabolism everything undergoes which comes through the gastrointernal tract. Thats the same as if you disable the enzymes in the liver with grapefruit juice just that you dont have to as the stuff doesnt go through the liver at all (in the first run, later for sure).
Cialis snorted/rectal/IV: Direct into bloodstream
Cialis swallowed: First through the liver (50% destroyed) then into the bloodstream
Letting it dissolve in the mouth is about equal to snorting.