How does a merge by crosswalk happen?
Merge by crosswalk happens when you are posting entities.
It happens each time you are posting the entity with the same crosswalk, as it already exists in the system.
Ideally, the platform should recognize that such crosswalk already exists and just update attributes of it. Sometimes the platform is so loaded that it cannot determine whether the crosswalk exists and in this case, the new entity is created and then merged with an already existing one by crosswalk (a very rare situation).
Also, merge by crosswalk does not happen when you are updating entities (e.g. _update endpoint).