twine around, check a current (with a weir)
笧 centers on the idea of entwining or obstructing flow: twining something around, or checking a current with a barrier like a weir.
笧 combines 竹 (bamboo) with 册, which likely contributes a phonetic element and may relate to the idea of a fence or barrier made from bamboo strips.
Bamboo 竹 strips woven together like the pages of a book 册 create a twining barrier that can check a current.
For サク, imagine a sack of bamboo strips used to build a weir: sack -> サク, and the bamboo twines around to check the water.