The default sync mode could be better explained in the docs, I agree
https://rclone.org/docs/#c-checksum
If you look at the docs for the --checksum flag it kind of implies how it works!
Anyway the default sync routine is
- check the size, copy if not equal
- check the modtime, skip if equal
- check the checksum
- if identical then set the modtime in the destination
- else copy