Synch scenarios are applied in two cases:
POST-requesting with getting a response
Invoking the scenario with Link scenario step, when you have to wait until linked scenario is completed.
In order to run the scenario synchronically with POST-request, you have to pick it in API-endpoint settings.
If API-endpoint has a Sync scenario, all the objects came with POST-requests (including requests from the form) will be processed in this scenario. The respond to the POST request will include the object after being processed in the scenario.
If you link scenario a-synchronically (by default), the object proceeds its way through the scenario. If you call the scenario synchronically, the object waits until the linked scenario is completed.