The model considers whether the utterance continuation after the disfluency is probable given the language model; the relevant bigram here is p(rr i+3 |w i ), continuing with p(rr i+4 |rr i+3 ).
under the incremental model, it is possible the utterance has only been read as far as token i + 3, in which case the probability p(w i+4 |w i+3 ) is undefined.