The principle
Keep people off-balance and in the dark by never revealing the purpose behind your actions.
The application
If they cannot see what you intend, they cannot prepare a defence. Reveal small, plausible motives that satisfy curiosity without exposing the larger plan. The work is not in lying — it is in giving people enough surface to stop digging.
The defensive tell — how to spot it being run on you
When someone is unusually open about a minor goal, ask what the minor goal would unlock if achieved. The decoy is almost always smaller and more sympathetic than the real prize. Watch for stated reasons that, even if true, would not justify the level of effort being put in.