Blog posts from Gareth, Imogen and Neil
Firstly, let’s clarify a few issues. The name. GUE calls it a “valve drill”. I have heard instructors from other agencies call them the same, but divers from all other agencies refer to it as a “shutdown”. There is a reason behind the alternative wording, but for simplicity's sake I am going to ignore the semantics and use the terms interchangeably.