writing brush, write, stroke
笔 centers on the tool and act of writing with a brush: a writing brush itself, the act of writing, and the strokes it makes.
笔 is the modern simplified form of 筆. The older form combines 竹 (bamboo) with 聿 (a hand holding a brush), reflecting the bamboo-handled writing brush. The modern form replaces 聿 with 毛 (hair), emphasizing the brush tip.
The top is bamboo 竹, used for the handle, and the bottom is hair 毛, used for the bristles. Together they make a writing brush: 笔.
For ヒツ, imagine a brush hitting the paper with a sharp sound: hits -> ヒツ, as the brush writes a stroke.