I think its a running stance. It most likely is what is known as weight transfer. If the rear end is squatted down, it tends to loft the front end, which puts more weight to the rear, which lends to better traction.

Its the same principle as a suspended drag car. Thats why they set the rear suspension to squat upon launch, and the front to loft. Traction.

you´re almost right there...
that stance isn´t actually anything the dog can do anything about... they´ve been selectively bred that way, for pretty much the reason you stated, but even more to increase their jumping ability, as to give them an advantage upon charging an enemy...
the downside of it is though that over 90% of the poor beggars get heavy rheumatism and/or arthritis in quite young ages, pretty much crippling them(at ´bout 8 years, which is "middle-aged" for a dog), which IMHO is cruelty to the animals(especially considering breeds of the past 10 years)...

but is he quicker that an aimed gun?

be nice use a tasser or tranck