If these vegetables are less than fresh, they may have lost some of their crispness. Soaking them in ice water returns some of this crispness because the food cells regain some of their lost water through osmosis. The water absorption re-swells the individual cells, causing them to press against each other and once again make the entire structure of the food more rigid and therefore crisper. The fact that its temperature is lowered by the ice water also helps crisp the tired vegetable, but to a much lesser extent than osmosis.
If the vegetables are fresh - as they should be - soaking will have the opposite effect. They will lose a bit of their crispness because of water buildup between the walls of the already plump cells. For some relatively dry vegetables like carrots, however, that scant loss is more than compensated for by the pleasant texture imparted by the extra moisture.
** Asian Recipes