I said that word aloud when I saw this construction in Gal. 5:25 this morning.
The chiastic form is not preserved in any English translation save the old ASV of 1901:
"If we live by the Spirit, by the Spirit let us also walk."
Lovely.
The meaning is: If the source of our life as believers is the Holy Spirit, the Holy Spirit must also be allowed to direct our lives so that we can make progress toward Christlikeness. The GNT puts it this way: "The Spirit has given us life; he must also control our lives." This verse is the finishing touch to Paul's argument in Galatians 5. Walking by the Spirit is the ONLY way to control the flesh and produce the fruit of the Spirit in our lives.
When the jars of clay remember that they are jars of clay, the treasure within gets all the glory -- which seems only fitting.