Truncate is not the same as shink. Was my explaination not clear?

Truncate just marks space inside the file for reuse. Shrink changes the size of the file.

Truncates occur on checkpoint in simple recovery mode and on log backup in full recovery mode.

SQL will not shrink a file without been told to do so.

In general, shrinking files is a bad idea. The file will just grow again, causeing file level fragmentation and taking up resources.

It's best to size a log file for the max it needs to be and then leave it alone.