This is the only Fang in the entire style whose name is explicitly built from two separate verbs joined by a middle dot (・) rather than one continuous compound — 伸 ("extend") and うねり裂き ("undulating tear") sit side by side as two distinct instructions rather than fusing into a single new word the way 切細裂き or 乱杭咬み do. That punctuation choice matters: it's the manga's own typographic way of saying this technique has two components happening at once — the blade lengthening its reach, and the strike itself moving in a wave rather than a straight line — where every other Fang collapses its action into a single unbroken verb-chain. It's a small structural tell that Ninth Fang, arriving late and against one of the series' most dangerous villains, is more mechanically complex than the six Fangs that came before it.
VIZ's "EXTENDING BENDY SLASH!!" earns particular credit for tracking that two-part structure rather than flattening it: "extending" carries 伸 cleanly, "bendy" carries うねり (undulating/swelling) about as well as a punchy shout-panel allows, and "slash" closes out 裂き. It's a looser, more colloquial register than a literal "Extending, Undulating Tear" would produce — "bendy" in particular is playground vocabulary next to うねり's more oceanic, wave-swell connotation — but the translation preserves the two-part construction the Japanese insists on, which is more than can be said for how this hub's underlying corpus first flagged the line: the automated bilingual pairing behind this entry recorded the English as "EXTENDING UNDULATING SLASH!!," a plausible-sounding but unconfirmed guess that direct inspection of the actual printed page (Vol. 18, correcting for this stretch's one-page offset) does not support. The real page says "bendy," not "undulating."
As the highest-numbered Fang confirmed anywhere in this corpus, Ninth Fang also marks a natural edge of what's verifiable about Beast Breathing's full technique list. Beyond this point, appearances of the style in later volumes (the final battle against Muzan Kibutsuji, Volume 19's fight against Upper Rank demons) reuse earlier Fangs or shift to unnamed, improvised strikes rather than introducing a confirmed Tenth Fang by name — see this hub's entry on Inosuke's improvised throw-slash for the clearest example of that shift from numbered technique to pure on-the-spot invention.