the dot is the kakekotoba — the pivot word. the slashes are its two readings. the form named your figure a thousand years ago and called it 掛詞, "hanging words."
Read with the words behind it, the pivot completes the phrase before — it closes the upper image, the kami-no-ku. This channel only exists looking back. \ = the BSS read, the verse finishing what came before.
One cluster of sound, carrying two denotations at once — matsu = "pine" looking back, "to wait" looking forward. Same syllables, two senses, and the two are sealed: the pine doesn't leak into the waiting. That seal is the air-gap, sitting inside a single word. The dot is the node both slashes pivot off.
Read with the words ahead of it, the same pivot launches the next phrase — it opens the lower image, the shimo-no-ku. This channel only exists looking forward. / = the FSS read, the verse starting what comes next. Mirror of \, opposite lean, same job reversed.
Waka bridges the dot. Haiku cuts it. The waka's kakekotoba joins across the junction — one word holding both sides. The haiku's kireji cuts at the same junction — a seam that splits two images yet still binds them. Same crossing point: read it as a weld and you have waka; read it as an air-gap and you have haiku.
That's why /.\ is cleaner than the upright figure. The two channels are mirror-symmetric — forward and back, doing one job reflected, not two different jobs — and the slashes can't be mistaken for each other, so the seal is written into the shape. Two characters. Explicit junction. No false "which way is up."