When the (S[ng]\NP 1 )/NP 2 passing is combined with the NP the buck, the lexical head of the NP 2 is instantiated with buck.
similarly, when the adverb just (s\NP 1 )/(s\NP) 2 is applied to passing the buck, a dependency between just and passing is created: because (s\NP 1 )/(s\NP) 2 is a modifier category, the head of the resulting s[ng]\NP is passing, not just (and no dependency is established between just and its NP 1 ).