-O ftrunc=tail: trim unused tail after processing actual data.

This is a hack actually, it's not much useful in everyday life.
It is for compressing files in place, exploiting stream compressors like this:

        xz -9c -T 4 < file.tar | tfcrypt -P -O ftrunc=tail - file.tar

, or more safer way (since tfcrypt does not buffer):

        tfcrypt -P file.tar | xz -9c -T 4 | tfcrypt -P -O ftrunc=tail - file.tar