How we change what others think, feel, believe and do
Let misleading information 'leak' out from your side.
Let them overhear you talking about particular (but false) needs or strategies that you have.
Leave documents on the table that they might read or copy.
Let something 'slip out' during conversation.
Have a person on your side 'sympathetically' tell them something.
In a negotiation I have my papers flat on the table with a highlighted section that can be easily read upside down.
We have a corridor conversation near where they are having coffee -- we get excited and voices get raised...
When people receive 'leaked' information, it can be very exciting for them as they believe they have a significant advantage over you. They will also be more likely to believe that it is true as it does not come from official channels.
This may well lead them to focus largely on the leak areas -- and consequently avoid other areas (where perhaps you do not want them to go).
When the leak proves eventually to be false (if they ever find out this), then they are unable to complain, for to do so would be to admit deceptive and possibly criminal behavior.