Mike from Zerto here.
For explanations of both processes, see the following links:
To answer your question more directly, if you select Reverse Protection when you perform a Live Failover, there will be a reverse delta sync after you commit the Failover. There is no reverse delta sync on a Test Failover, because there is no reversing the protection for a Test Failover. A Test Failover is meant to be an isolated test, so production is never affected, and there would be no replication back to the production site.