maybe its a an "down under thing",
have a look here Glossary of nautical terms - Wikipedia, the free encyclopedia
"knock" yourself out looking for "knock", then look at the entry marked "header"
Ive never heard the term "knock" being used in that content. If the winds changes the boat
is "headed", hence the crew are always shouting "header" coming or "lift coming" to the trimmer and helm
i suspect knock is either slang, or developed from lack of nautical knowledge and has entered some common usage