boundary between paddies
畛 centers on a physical boundary or dividing line, specifically the raised path or ridge between rice paddies.
畛 combines 田 (rice field) with 㐱, which likely contributes the sound and may relate to dividing or marking. The exact historical development of 㐱 is uncertain.
The left side 田 is a rice field, and the right side 㐱 looks like a person walking along a narrow raised path. Picture a farmer balancing on the ridge between flooded paddies.
For シン, imagine a thin shin-high ridge separating the paddies: shin -> シン, and you walk carefully along the boundary.