Rsyslog is backwards-compatible with Syslog configuration files.
The syslog.conf man page has:
You may prefix each entry with the minus ``-'' sign to omit syncing the file after every logging. Note that you might lose information if the system crashes right behind a write attempt. Nevertheless this might give you back some performance, especially if you run programs that use logging in a very verbose manner.
but I could not find aything about the - sign in man rsyslog.conf.
What does rsyslog do when if reads - in the config file?
It has the same function. Docs (cached version, since the original is returning an error right now)