If you have recently come across people wildly waving their arms and looking somewhat frenzied, you may have been witnessing "consensus-based decision-making". I first saw it in action last month while I was giving a book talk at York University. As I was speaking, an audience member raised his hands and started waving at me as if at a three-year-old. Soon, others joined in; some waving with their arms up, others with them down, a few rolled their arms as if gripped by a collective spasmodic episode.

Bewildered, I asked what was happening. "These silent gestures all mean something," said a student. "If I raise my hands waggling, I agree; lowered waggling hands means I disagree. If I raise one fist I'd like to speak, two and I need to speak urgently. A technical point – like 'the building is burning down' – is raised by a T-sign, and when you roll your arms like I did just then, I'm bored and want the speaker to get to the point." I got the point.

The purpose of the process is that every participant is honest – as you'd expect since it originated in the Quaker movement. Consensus decision-making isn't designed for book talks, of course – generally authors prefer it if you wait until we've left before complaining. But it has now become a ubiquitous part of grassroots campaigning. Every meeting of UK Uncut, the tax justice movement, and many student protest meetings are now using it.

When young people laid siege to Fortnum & Mason a few weeks ago, and were surrounded by police, they employed "consensus" to work out what to do next.

"We used the stairwell in the middle of the store – between teas of the world and fruit and veg – so that everyone could see each other," explained one of the activists (later arrested). "There were two sessions that afternoon lasting about half an hour. It's participatory and civilised – because everyone has to agree."

And precisely because of that, there is an obvious drawback. "It can take, literally, an entire weekend of meetings to make a decision," explained the guy in handcuffs. It turns out that sticking it to the man takes ages if you fight him with jazz hands.