to sort out the stalks used in divination, to fold
揲 centers on handling stalks in a ritual divination process, specifically sorting or counting them. The folding sense is a separate, less common meaning that likely arose from the physical action of bending or arranging the stalks.
揲 combines the hand radical 扌 with 枼, which likely contributes the sound. The character is associated with the act of sorting yarrow stalks in ancient Chinese divination, but the exact historical development of the folding meaning is uncertain.
The hand radical 扌 beside 枼 (which looks like a bundle of stalks on a wooden frame) suggests using your hands to sort or fold ritual stalks.
For セツ, imagine a set of divination stalks being sorted by hand: set -> セツ, and your hands carefully arrange the set.