The content below is for Apache Syncope 1.2 - for later versions the Reference Guide is available.
Basically, the push is a sort of synchronization mechanism used by Apache Syncope to propagate a filtered set of user/role/membership data to external resources.
Push can be "full" only: all the data matching the configured filter (potentially the same set of data) will be sent to the external resource at each push task execution.
All the entity (user/group) data involved by a push are retrieved locally and compared with remote ones before sending out.
An entity to be sent out can be:
By default, Syncope will propagate all the unmatching entities for provisioning (without linking entities and resources) and will update all the matching ones.
By the way, a different behaviour can be configured working with.
Unmatching (corresponding user not found on external resource):
Matching (corresponding users found on external resource):