tempest back doors

Perhaps I am stupid. I don't know how one would go about modifying application software to include a 'back door' that would presumably enhance its suceptibility to TEMPEST attacks. Isn't tempest all about EM spectrum signal detection and capture?

You have your code drive a bus with signal. The bus radiates, you 'TEMPEST' the signal, game over. Back in the 60s folks programmed PDPs to play music on AM radios. Same thing. Dig?