The form of a wave (luk) is dependent upon the classification of the keris:- long, slow waves point towards one classification, short, deep waves point towards a different classification.
I've used the word "classification" as a substitute for "tangguh", it doesn't mean "tangguh", but it is a more objective way of thinking about a blade.
See:-
http://www.kerisattosanaji.com/keristangguh.html