Wikipedia
Funkspiel was the name of a Counterintelligence operation carried out by Nazi Intelligence during Second World War. Captured Radio Operators in France were to forcibly send false messages to British Intelligence.
It allowed Nazi Intelligence to intercept Allied Military Information, convey Disinformation to the enemy and actively fight Resistance movements. By doing so Nazi Intelligence made the pretence of being the French resistance with a script written for the enemy by the Gestapo or the Abwehr.
The last false message exchanged with London in this operation was: "Thank you for your collaboration and for the weapons that you sent us". However Nazi Intelligence was not aware that British intelligence knew about the stratagem for at least two weeks prior to the transmission. If Nazi Intelligence was able to gain some benefit from the operation, ultimately the stratagem was used against it. From May 1944 onwards the operation was not a success.
A similar Funkspiel Technique called Operation Scherhorn was executed by the Soviet NKVD against Nazi Secret Services from August 1944 until May 1945.
Funkspiel also referred to a Technique used by U-boat Radio Operators in which the frequency of transmission was changed consecutively to confuse Allied Intelligence with the objective of picking up enemy transmission in the original channel.