When a melody is performed by two performers, with each one playing it in reversed directions, we say that they are advancing in "mirror motion".
There are two principal options for performing mirror motion: (1) the music is played as if a mirror is placed at the end of the melody - in this way, one performer plays the music as it is written, while the other plays it from the end to the beginning; (2) the mirror is "placed" just beneath the notes - the second performer moves in a contrary direction to the first performer, ascending when he descends and vice versa.
This is exactly what occurs in bar 14 of the first movement of Beethoven's Fifth Symphony.
