The Sonic Effect is a sound effect that Jimmy made as a lead in to Heartbreaker for a number of performances on the Spring US tour in 1970.
Like the Drone that was used in the summer of 72, the Sonic Effect served to hypnotize you briefly and allow the guitar to snap you out of it at the beginning of the song that it lead into.
It was cool, but ultimately retired at the end of that tour because, when the set was redesigned that summer, Heartbreaker was played directly after Immigrant Song for the next two years.