https://kb.fortinet.com/kb/documentLink.do?externalID=12069
FortiGate Infrastructure 7.2 Study Guide (p.264): "...then FortiGate might drop interesting traffic because of the absence of active SAs. To prevent this, you can enable Auto-negotiate. When you do this, FortiGate not only negotiates new SAs before the current SAs expire, but it also starts using the new SAs right away." "Another benefit of enabling Auto-negotiate is that the tunnel comes up and stays up automatically, even when there is no interesting traffic. When you enable Autokey Keep Alive and keep Auto-negotiate disabled, the tunnel does not come up automatically unless there is interesting traffic. However, after the tunnel is up, it stays that way because FortiGate periodically sends keep alive packets over the tunnel. Note that when you enable Auto-negotiate, Autokey Keep Alive is implicitly enabled."