Hard crack stage will make the candy you put into the cold water hardened before it can hit the bottom of the glass you are using. This is the way my Grandmother always tested her candy; I live at a high altitude and a candy thermometer doesn't always work right so I do it the way I learned. I always use a clear smooth glass ( a pint jar would work for this) so that I can see the candy in the water. If it turns cloudy as you put less than a teaspoon in, it is no where near ready. BTW, this is how I test the sugar mixture for popcorn balls too. Gobo, you can use a clear butter extract (like what is used in wedding cake icings) in that recipe to give it a melt in your mouth taste!!