The folding of the legs of Arich Anpin was what first gave the broken vessels the power to ascend, making possible all their subsequent ascents. The truth is that the vessels were dependent upon these legs. At first they were sustained through them down below. Then, when the legs were folded up, this gave the vessels the power to return and go back up. It is in this sense that Arich Anpin (of the first Atzilut) is the root of everything. The legs were the part of Arich Anpin in which the vessels were rooted during the time of their descent. Then afterwards (as a result of the folding of the legs) they also became rooted in Chessed-Gevurah-Tiferet as the extent of their repair increased.