[hyyypr]

This is interesting.

Yelp has also started doing something along these lines selecting quotes from reviews in which a word that occurs frequently has been found.

[benten10]

I would suspect that instead of word frequency, they likely use TF-IDF (or even better, BM25[1]) or LDA [2] to identify relevant keywords to highlight.

1. https://en.wikipedia.org/wiki/Okapi_BM25

2. https://en.wikipedia.org/wiki/Latent_Dirichlet_allocation