Someone poses as a sympathetic ally to people resisting a change, draws out their plans, allies, and weak points, and uses what they learn to undermine the effort.
An example
When a newspaper group announced it would merge the newsrooms of its two largest papers, a managing editor at the smaller one organized senior editors to propose a different structure to the board. A deputy editor from the group's corporate staff, known for complaining about the merger in private, began joining their after-work meetings. The deputy was warm and indignant, and asked practical questions: which board members they had approached, which editors would sign, when they planned to send the proposal. The managing editor, tired and grateful for support, shared the list. Two weeks later the group's chief executive met separately with three of the signatories and offered each a senior role in the merged newsroom. Two withdrew. The proposal reached the board with four names and a week less preparation than planned, and the board approved the merger as announced.
Why it works
It works on all four. The false friend offers what a resistor under pressure lacks, validation and the prospect of a bigger coalition, which makes disclosure feel good, and establishes themselves as one of the group, which makes it feel safe. Holding back starts to look paranoid, and refusing the alliance costs the resistor a needed ally. It depends on a cover that past behavior does not contradict, and it works best on resistors who are isolated, stressed, and short of support. It fails when resistors compartmentalize by habit or when the friend asks too much too fast.
The thinking behind it
Sun Tzu (Sunzi) The Art of War (date uncertain), read for its maxims, holds that victory depends on foreknowledge, which comes from people who know the opponent's situation, and its chapter on spies sets out several classes of agents (chapter 13). A false friend is a source of that foreknowledge planted inside a resisting group. The defense follows from the same text: limit what any single new ally can learn about plans, names, and dates.
Erving Goffman Erving Goffman's The Presentation of Self in Everyday Life (1956) describes performance teams: people who share an interest in a performance succeeding and keep their preparations in a back region the audience does not see. A group resisting a change is such a team, and its plans are its back region. A false friend gains entry by performing membership. Checking a new ally's past conduct against the sympathy they profess tests the performance before the back region opens to them.
Warning signs
Unusually specific questions about plans, allies, and timing come from a new or newly sympathetic ally. The ally's past behavior or position does not quite fit the sympathy they profess. The questions arrive when resistors are isolated and under stress. Shortly after, the other side seems to know your plans and approaches your allies one by one.
How to defend yourself
Share plans on a need-to-know basis, even with sympathizers, and make that a habit for the whole group so no one feels singled out. Check a new ally's past behavior and position against the sympathy they profess. Notice when questions get too specific too fast, especially about names and dates. Give new allies a useful role that does not require the full list. Keep your core coalition small and in direct contact, so the other side's approach to any member is reported back quickly, and agree in advance on what each of you will say if approached.
How a leader shuts it down
Leaders running a change can refuse to act on information gathered this way, and say so, since its use damages trust long after the change is done. Provide open forums where resistors can raise objections without needing to organize in private. Treat exposure of a planted ally as a serious conduct matter.