Work to Rule is an employee-driven action where employees follow their written work instructions “to the letter”. If something is not in their work description, they do not do it. They take their breaks at precisely the allowed times, and never show up early, or late. They follow all safety protocols, to the letter, even if it slows down productivity. Their collective agreement, if there is one, is followed to the the letter as well.
If Work to Rule is organized by a union, it is often considered a form of Strike, and therefore the rules for strike are in force (giving notice, etc.)
However, if it is unorganized (whisper campaign, perhaps), then the union has plausible deniability, as they did not promote it–the employees are just following their collective agreement.
Disclaimer: these are my understandings only, having been a union rep, organizer and strike organizer back when I was in the work force. Make sure you understand the laws in your province or state, and your collective agreement.