Not quite - the transaction log will be truncated after a log backup has occurred *and* there are no open transactions in the previous VLF's. The log also cannot be truncated if you have an availability group - and the transactions have not been hardened on the secondary.
If your question is related to the size of the transaction log then you should know that truncation <> shrink, meaning the log file will not shrink upon truncation. All truncation does is marks a VLF as reusable within the file and SQL Server rolls over to the beginning of the file once it gets to the end. If all VLF's are in use when SQL Server fills up the last available VLF - then the transaction log will grow.