Coal mining is a dangerous activity. Deep underground, it is difficult to detect the presence of toxic gases. Coal produces carbon monoxide when it burns. Methane gas, formed during coalification and emitted during mining, carries both suffocation and implosion risk.
At the turn of the 20th Century, John Scott Haldane suggested a novel (if horrific) safety measure - canaries. Physiologically they served as effective early warnings on air quality, as they'd get sick before the miners did. This allowed workers to fit their self rescuers and evacuate the area. The canaries were eventually replaced with electronic devices in the late '80s, around the time when Maggie Thatcher's government was shutting the mines, decimating communities across the country.
Since then, "canary in the coal mine" has been a synonym for early-warning systems.
A warrant canary is a similar technique that warns users against circumstances where the service has been investigated by law enforcement authorities, and there is a legal restriction from reporting on this. A notice is publicised well before any investigations, for example, "The FBI has not been here". If this notice is ever removed, it's not categorically stating that the event has now happened. Still, it can be interpreted as being indicative. Users may then change their behaviour accordingly. Apple and Reddit have previously published and removed warrant canaries.
Recently in longer-form communications and in certain documents I really want someone to pay attention to, I've devised a similar technique that I've called an attention canary.
The idea is to carefully place a certain piece of information so that you can work out who's paying attention. The human attention span wanes between 10-20 minutes, so it's somewhere in this ballpark you're targeting. People will pick up on the odd thing, and their attention is jolted.
At a recent internal presentation, I added my birthday onto a slide detailing the timeline of a new product rollout. It's whimsical and entirely non-harmful. After the presentation, I received a few messages remarking on it! I can use this as a sense check on levels of audience engagement.
Alternatively, this could influence document design, particularly the placement of figures, charts, headings, bullets or images. Break up long blocks of text, and change the pace of the document. Grab back the user's attention by summarising the crucial details or preparing them for the next burst of focus. This pattern can also be used between logical sections of a presentation, chapters in a book, and so forth. The canary is a call to action to reorient, but it's also a jumping-back-in point.
As a parting exercise, consider the communication activities you participate in as a presenter or audience. What interests you? What bores you? What keeps you engaged? How could you restructure that complicated explanation to be more accessible?