The “stress measurement spot” is a tiny pocket containing liquid crystals, which change colour depending on temperature.
Liquid crystals are molecules that arrange themselves in different patterns, depending on temperature. Each pattern reflects light in a different way, which we perceive as a colour change. They can be made sensitive enough to detect changes as small as 0.1 degrees Celsius.
The temperature of your hands is a good indicator of your mood. Happy or excited people tend to have hot hands, while stressed or nervous people have cold ones. This is because stress causes the body to remove blood from extremities. Alternatively, when you're happy or excited, blood flows into your extremities, slightly increasing their temperature. Therefore cool hands often indicate high stress and warmer hands indicate relaxation (i.e. calmness).