Some gliders also have flaps which negative settings. As Fabo already mentioned, they are used for decreasing drag when flying at higher speeds.
As you all know, a slim airfoil equals little lift and drag, and vice versa. Also, at higher speeds, more lift and more drag is produced.
So, with negative flap settings, you reduce drag and lift, which then can / must be compensated by flying faster.
I hope I managed to express myself understandably. If not, let me know, I'll rewrite this when I'm not about to get some sleep
And, I should type faster . . . .