Define a connector for some kind of persistence unit (LDAP, SQL, does not matter) and an associated resource.
Create a set of provisioning rules for that resource, especially define a mapping with an attribute that is nullable on the resource and Syncope.
Create a pull task on that resource and enable create and update (set also matching and unmatching rules accordingly).
Pull objects from the resource in order to create them on Syncope, then try to reset values on the external resource for some mapped attribute and re-pull in order to update obejcts. You'll see that the values are not reset on Syncope.
an test case is explained is here