flail
耞 centers on a farming tool used for threshing grain: a flail, which is a wooden staff with a swinging stick attached, used to beat harvested crops to separate the grain.
耞 combines 耒 (a plow or farming tool) with 加 (add, increase), likely indicating a tool added to the farming process for threshing. The exact historical development is uncertain, but the components suggest a semantic compound related to agricultural implements.
The left side 耒 represents a farming tool, and the right side 加 means 'add'. Picture adding a flail to your set of farming tools: the wooden handle with a swinging stick attached is used to beat grain.
For カ, imagine a farmer using a flail and shouting 'Ka!' with each strike: the sharp sound 'Ka' cues カ while the flail threshes the grain.