[ Etymonline :] from Latin versus "a line, row, line of verse, line of writing,"
from PIE root *wer- (3) "to turn, bend" (see versus). The metaphor is of plowing, of "turning" from one line to another (vertere = "to turn") as a plowman does.
How does the metaphor of plowing explain the semantic shift from the PIE root to Latin? I can understand that the act of scrutinising each line of writing can be metaphorised by plowing each row, but then how does proceeding from one line to the next connect with turning in plowing? A reader would descend ocularly from the current line of writing to the next line; it sounds strange to describe this as 'turning' from one line to the next.