Let up and Narrow down

Let up verb - To come to an end.

Narrow down and let up are semantically related In some cases you can use "Narrow down" instead a verb "Let up".

Narrow down verb - To reduce in size or volume by or as if by pressing parts or members together.

Let up and narrow down are semantically related in decrease topic. Sometimes you can use "Let up" instead a verb "Narrow down", if it concerns topics such as grow less or make less.