what's downward causation?

this is a good definition: 

Downward causation can be defined as a converse of the reductionist principle above: the behavior of the parts (down) is determined by the behavior of the whole (up), so determination moves downward instead of upward. The difference is that determination is not complete. This makes it possible to formulate a clear systemic stance, without lapsing into either the extremes of reductionism or of holism: the whole is to some degree constrained by the parts (upward causation), but at the same time the parts are to some degree constrained by the whole (downward causation). 

Snow crystals have a strict 6-fold symmetry, but at the same time that each crystal has a unique symmetric shape, the symmetry of the crystal (whole) is clearly determined by the physico-chemical properties of the water molecules which constitute it. But on the other hand, the shape of the complete crystal is not determined by the molecules. Once a shape has been formed, though, the molecules in the crystal are constrained: they can only be present at particular places allowed in the symmetric crystalline shape. The whole (crystal) constrain or "causes" the positions of the parts (molecules).

let's review it:

(1) the symmetry of the crystal six-fold (whole) IS determined by physico/chemical properties of water molecules.
(2) the shape of each crystal (part) IS NOT determined by the molecules, they can only be present at particular places allowed in the symmetric crystalline shape.

so, (1) downward causes (2), but (1) is constrained by (2) (the constrain means upward causation).