Achas-beta is a method of alphabetic transformation wherein the first 21 letters of the alef-beit are initially divided into three equal groups of seven letters, which are then tripled together in "direct order":
The first letter in each group--alef, chet, and samech; the second in each group--beit, tet, and ayin; etc. The final letter tav is added to the seventh triplet. The transformation pattern is one of "revolution" within each triplet, i.e. alef transforms into chet which transforms into samech which transforms into alef, etc.
The name achas-beta is a reference to the first two of these transformation triplets.
This is the alphabetic transformation whose elements correspond to the sefirot within the partzuf of Ze'ir Anpin (the seven emotive attributes).