Why do dogs shake their leg when you scratch them?

I did not know the answer to this until today.

ANSWER:

Tickle a dog's tummy, and you could get a knee-jerk reaction.
There is a certain area on a dog's body that is referred to as the
"saddle region." It vaguely consists of the back, sides and flanks of
the pooch. Rubbing or scratching a dog in the saddle region (the exact
spot varies according to the individual pet) will cause the hind leg
to simulate a scratching motion. This is called the "scratch reflex."

The dog's spinal nerves pass all the way to his chest and abdomen, and
stimulation of these nerves will cause a feeling of "itchiness"
(similar to the irritation of a flea) somewhere on his body, causing
the rear leg to involuntarily attempt to scratch it. The leg extends
to different lengths or reaches to varying heights depending upon the
area that is stimulated.
Veterinarians who suspect spinal or neck damage in canines use the
scratch reflex as a diagnostic tool, much like when doctors tap humans
just below the kneecap to test nerve reactions.”