In the 68th measure of the first movement of his Requiem in C minor, Haydn employs an Augmented 6th chord in the form of a German 6th. The chord is spelled out in the piano accompaniment and works in tandem with the SATB voicing. In the choir, the spelling is A in the bass, C in the tenor, F# in the alto and C in the soprano. Underneath the choir, the piano leads into the chord with a B natural, D G# ornamentation, then settles into the German 65 chord with the necessary le do and fi.
To avoid parallel 5ths the German 6th is followed by a V64 chord that resolves and eventually heads back to the tonic. The German 6th is also preceded by a V65-53 chord that harmonically prepares for the German 6th and also prevents the presence of parallel 5ths. In regards to the use of the chord. The dissonance of the augmented 6th adds to the somber mood of the requiem. In this first movement, Haydn is establishing the themes with which he will present his mournful piece. In this particular phrase, the choir is singing "Kyrie leison" at which point, the German 6th gives the dissonance with which Haydn hopes to instill a true sense or mourning, sadness and very unsettling feeling.
In terms of performance, it would be best in my opinion to emphasize the German 6ths so that the listening audience fully comprehends the mood of the piece which I feel the German 6th has a lot to do with in that respect. In addition, in terms of micromanaging, I would make the voice part singing the fi sing out over the other voices to really emphasize the dissonance and clashing that help the mournful essence of Haydn's requiem shine through.