insert
揷 centers on the action of inserting something into a space or opening. The core idea is pushing or placing an object into a target.
揷 is a variant form of 插. It combines 扌 (hand) with 臿, which likely contributes the sound and relates to the action of inserting or piercing. The exact historical development of the right component is uncertain.
The left hand 扌 holds something, and the right side 臿 looks like a tool being pushed into a slot. Picture a hand inserting a key-like object into a narrow opening.
For ソウ, imagine inserting a sewing needle into fabric: sew -> ソウ, and your hand carefully pushes the needle in.