A sink-fold is sort of a reverse-fold in 3D. In a sink-fold, a part of the middle of the paper is reversed back into the paper (and may be reversed back out again, if you want this).
Crease firmly with either a mountain- or a valley-fold to get the line of the sink-fold. Spread the center of the paper and push the upper portion downward. The upper portion of the paper will flatten and then turn inside-out. Then flatten to form the sink-fold.
Click here for folding tips.