There are 2 parts of the function at. The first part of the function adds values and the second part deletes them. To be honest, I’m not 100% sure how the develops example does the work, I’ve never looked at it closely. My plugin does things quite a bit differently.
I have another example here that does it in yet another way https://github.com/Hube2/acf-filters-and-functions/blob/master/acf-reciprocal-relationship.php
