The word for this is "esoteric". An "esoteric ritual" is one that's easily kept secret because it's hard to learn even if you try to explain it. Like, you can'

The word for this is "esoteric". An "esoteric ritual" is one that's easily kept secret because it's hard to learn even if you try to explain it. Like, you can't learn to drive a car or play a violin by reading a book about it.