The Luhn summarizer returns the most significant sentences of a text by :
1 - Calculating frequencies ratio of significant terms (discounting ones with ratio outside an arbitray range, ie "normalized").
2 - Calculating each sentence rating ((number of significant terms)² / (greatest distance between two significant terms)).
3 - Sorting sentences according to their weight and returning the first n of them.
Reference : https://ieeexplore.ieee.org/document/5392672?arnumber=5392672