Ach-bi is a method of alphabetic transformation wherein the alef-beit is divided into two equal groups of eleven letters. Within each group, the letters pair "reflectively"–the first with the last, the second with the second-to-last, etc.
The name ach-bi is a reference to the first two of these transformation pairs: alef-kaf and beit–yud.
This is the alphabetic transformation whose elements correspond to the sefirot within the partzuf of knowledge (da'at.)
The three transformation systems, al-bam, at-bash, and ach-bi (corresponding to wisdom (chochmah), understanding (binah), and knowledge (da'at) form a "transformation ring," which means that if you transform a letter using all three you will come back to your original letter. E.g., alef in al-bam transforms into lamed; lamed in at-bash transforms into kaf–, which using ach-bi, transforms back to alef.