I was at the mall today with my family and we ate lunch in the food court. My 11 year old had ordered sesame chicken from Panda Express and while eating her lunch she asked me "Daddy when you make chicken it isn't soft like this" after that she began to tell me that i needed to read a cook book haha. Once I got home i started researching the topic, below you will see what i found.
It is a common Chinese cooking technique called velveting the meat.
Basically, the meat is marinated in egg whites and cornstarch (other optional seasonings may be added, depending upon what flavor you wish to achieve), then refrigerated for about an hour before cooking. This helps to create a barrier between the direct heat source and the meat, so that water does not escape while cooking. It leaves the meat moist and tender.
Here is a slightly better explanation, that I've found:
The largest effect is that the velvet adds a thin, clingy coating to the outside of the meat. When introduced to heat, that's providing a barrier to the movement of thermal energy into the meat proteins. The proteins in the egg are denaturing, the starches are gelatinizing, and that absorbs some of the energy that would otherwise have gone into your meat. The presence of sheer additional mass from the velvet also means that it simply takes more energy to raise the overall temperature. This means that the temperature increases more slowly, and can be better controlled (sort of like sous vide cooking).
As the starch gelatinizes, it's also forming a moisture-resistant barrier around the outside of the meat. That could very possibly prevent moisture from leaking out of the meat as it's squeezed out of cells with denaturing proteins. Ordinarily it would leak out into your poaching liquid (or wok if you're stir-frying, where it would rapidly boil off) but now it's trapped in the pieces of meat. This is also somewhat analogous to sous vide cooking, or maybe to poaching in oil - the food being cooked stays moist, because the water is trapped inside (by a bag in sous vide, by hydrophobic oil in oil-poaching) and has nowhere to go.
I've never personally been in Panda's kitchen, but can almost guarantee the stir-frying cooking technique used for most of their dishes generally involves a very large fire (like the kind that comes out of the Bat mobile's tailpipe), a carbon-steel wok and a tossing/flipping technique that looks very similar to this
