What is the secret of the handkerchief?

In Lois Lowry's novel, Number the Stars, Annemarie finds out that the handkerchief that her Uncle uses is a special handkerchief that has something on it that confuses the noses of the German soliders' dogs. The handkerchief saves those on the boat because the dogs can't smell them. Uncle Henrik tells her this secret and says that Peter worked with some scientists to come up with the special additive that masks their scents.