“Lining out” is the practice of singing or stating the upcoming lines of a hymn for the congregation by a song leader or preacher. Typically, the words are stated quickly, and then sung in a drawn-out, slowly rhythmic moan.
Ever since finding it for a collaged-together program of gospel videos I curated for Portland's historic Hollywood Theatre three and a half years ago, I've been mesmerized by the above performance of C.P. Preston's hymn "A Charge to Keep I Have," shot at Evergreen Baptist in Fort Lauderdale 15 years ago.
There's a recording of Leadbelly singing the song, "Look Away to Heaven," and in introducing the tune, the singer says that in his experience at the Baptist church he attended as a child, it was “sisters in the Amen corner” who performed the role of introducing the lining hymn.
Today lining hymns are the principal form of singing—purely a capella lined-out hymns—in the Southern Old Regular Baptist church; there are several excellent recordings of this material on Folkways. The tradition seems to have been equally strong in both black and white churches, from what I can tell.