By taking advantage of dual-fiber testing, PAIR MODE is the fastest way to certify a pair of fibers simultaneously, since industry cabling standards such as the TIA-568 only require fibers to be tested in one direction. In other words, the direction of the test really doesn't matter.
Since pairs are typically numbered in odd/even combinations such as 1&2, 3&4, 5&6, and so on, the odd numbered fibers (1,3,5...) are tested in one direction, while the even numbered fibers (2,4,6...) are tested in the opposite direction.