What is ‘hymn lining?’
Lining out, also called hymn lining, is a form of a cappella hymn-singing or hymnody in which a leader, often called the clerk or precentor, gives each line of a hymn tune as it is to be sung, usually in a chanted form giving or suggesting the tune. It can be considered a form of call and response.
The practice of lined-out psalmody was first documented in England by the Westminster Assembly, which prescribed it in 1644, though only for those congregations with an insufficient number of literate members or printed psalters. It became however the norm in English Dissenting churches of all levels, and American ones as well, even after psalters became more readily available.
Wiki
The video places emphasis on the use of hymn lining in black churches and is wonderfully done. God bless.