I believe it was during an interview when he explained the technique succinctly while talking about illegal spying against his campaign by the Manchurian “administration” that preceded his. He mentioned that he doesn’t do email and prefers to talk on the phone but always assumes there are third parties listening.

Please link if you find the clip.

Edit: He also alluded to the technique in an another interview while talking about the Ukraine phone call.

I believe it was during an interview when he explained the technique succinctly while talking about illegal spying against his campaign by the Manchurian “administration” that preceded his.

Please link if you find the clip.