Johnson explains the title's use of the trombone by discussing the vocal and rhetorical qualities of a preacher he had recently heard who, he felt, exemplified the compelling and persuasive nature of the folk preacher, naming the trombone as "the instrument possessing above all others the power to express the wide and varied range of emotions encompassed by the human voice — and with greater amplitude."
Johnson uses the trombone as an analogy to describe a preacher's vocal and rhetorical qualities, emphasizing how it can express a wide range of emotions—similar to the human voice—and has enhanced volume.