Leapa

"Say my secret word is GO," Leapa said. "I decide the reader will take every third letter. So I put G in the first slot, then two junk letters, then O, then two more junk letters: G-x-x-O-x-x. I fill the junk with anything — G, T, R, O, P, L — 'GTROPL.' It looks like gibberish, or even like a real-ish nonsense word. But a solver who knows the rhythm reads the first, the fourth, the seventh — every third — and 'GO' steps right out of the noise. The message was there the whole time, just wearing a costume of extra letters."

"The trick to making it fair," Leapa said, "is the solver has to know the skip — every third, every fifth, whatever I chose. That number is the key, like Kaesa's shift-number. And I have to be careful to place my real letters at exactly the right spacing — if I put one real letter a slot too early, the whole rhythm drifts and the message falls apart when they read it. So I build it on a little grid: real letter, gap, gap, real letter, gap, gap — counting every position. Done right, it's the most invisible hide there is: a rival can hold the clue in their hand, convinced it's nonsense, while the answer sits inside it in plain view. Hidden not by scrambling, but by rhythm."

The member tried hiding NO with a skip of two — every second letter. "N in slot one, junk in slot two, O in slot three, junk in slot four: N-x-O-x. 'NBOK.'" They read every second letter starting from the first: "N... O. It's there!" Then they realized: "Wait, I have to tell my solver to start at the first letter and take every second. If they start at the second, they'd read the junk." Leapa nodded. "Exactly — say the start and the skip. Get the rhythm right and it's invisible."