to restrain, to dam a stream and change its direction, mound
垔 centers on blocking or holding back a flow, especially water, by building up earth. The mound sense is the physical earthwork used to restrain or redirect the stream.
垔 combines 覀 (a variant of 西, possibly representing a covering or obstruction) with 土 (earth), suggesting earth used to block or dam. The exact historical development is uncertain, but the component relationship is consistent with the meanings of restraining or mounding earth.
Picture a mound of earth (土) piled under a covering (覀) to dam a stream and force it to change direction. The earth restrains the water.
For シュ, imagine a shoe (シュ) being used to plug a leak in the earthen dam. The shoe restrains the water just like the mound.