Set the log file location
If you made the backup copy of the Synchronization Service Configuration File, then you can rename the tracing file to miiserver.exe.config._TracingEnabled. Once you do that, then rename the backup copy back to miiserver.exe.config. If you do this, then you should be able to easily toggle back forth to enable/disable tracing.
Anytime you change the Synchronization Service Configuration File, then you will need to restart the FIM Synchronization Service.
ADDITIONAL FIM TRACING ARTICLES
How to enable FIM Service Tracing
How to configure FIM Portal Tracing
Enable Tracing and Viewing Trace Logs
Enable Verbose Tracing for the FIM Portal
Current Forefront Identity Manager 2010 Resources
Current Certificate Lifecycle Manager Resources
GalSync Resource Wiki
PCNS-Password Synchronization Wiki
Self-Service Password Reset (SSPR) Resources