What are Hot Water Doughs
from http://www.seriouseats.com/

With most Western brseads and pastries, cold or room temperature liquid is added to flour before kneading it. There are two major proteins in flour, glutenin, and gliadin. Here's the thing: they're kinda kinky (no, not in that way), and when they get wet and are rubbed around through mechanical action (like kneading), they tend to stretch out and bind with other glutenin and gliadin molecules, forming the stretchy protein matrix known as gluten.

Gluten is what gives dough structure, and the more it's kneaded and worked, the tighter and more elastic it becomes. A ball of well-kneaded cold water dough will spring back if you press it and contract if you stretch it. This is why, for example, pizza dough is extremely hard to roll out until it's had at least a couple hours to rest and allow this gluten to relax. The level of chewiness and stretch you get from a cold water dough is directly related to how vigorously its kneaded, and how long it rests.

Hot water doughs—the type used to make Chinese scallion pancakes, dumpling wrappers, and several other Chinese pastries—work a little differently. By adding boiling water directly to flour, you actually end up not only denaturing the proteins, but smashing them into small pieces. Some degree of gluten can still form, but because cooked proteins aren't nearly as stretchy or clingy as raw ones, you won't get anywhere near the stretch or elasticity of a cold-water dough.

If airy, hole-filled bread is your goal, this is a bad thing. If, on the other hand, you're looking for tender dumpling wrappers or scallion pancakes with just a bit of tug and chew, that's precisely what you want.

The beauty of a hot water dough is that as you can see, it doesn't bounce back as much as a cold water dough does. This makes it extremely easy to work with and roll out. A positive boon when you've got 50 dumpling wrapper skins to form, or when you're making scallion pancakes. The texture can be described almost as a thick paste. Even better, because it's got so little gluten development, you can work with it almost straight out of the fridge, making it easy to prepare your dough in the morning, throw it in the fridge, come back just before dinner, and roll out as much or as little as you need for your meal.

Recipe for Hot Water Dumpling Wrappers