helm sources have a slot named match-part which, IIUC, is the part of the transformed candidate that will be matched. In particular, the source helm-files-in-current-dir-source has the following match-part:
(match-part :initform (lambda (candidate)
(if (or helm-ff-transformer-show-only-basename
helm-recentf--basename-flag)
(helm-basename candidate) candidate)))
This source also has fuzzy-match set to t. With helm-ff-transformer-show-only-basename set to t, I still see this source, when used as part of helm-for-files, doing fuzzy-matching on the full file name and not just the basename. For example, abc matches the file aaa/bbb/ccc. I had assumed match-part would transform the candidate aaa/bbb/ccc to just ccc, which abc would fail to match even with fuzzy matching on. What am I doing wrong?
The fuzzy match function which kicks in for the source helm-files-in-current-dir-source is the default helm-fuzzy-match.