What does tying the arm around the elbow do? ?

I'm going to assume that you are referring to a cord or piece of elastic band to cut circulation. Cutting off or reducing the venous return system which runs towards the heart, will cause the veins to become engorged with blood. These enlarged veins are easier to locate and stick with a hypodermic needle.

Anxiety disorders, attention deficit hyperactivity disorder (adhd, add), and autism spectrum disorder (asd). (source: )

There is tumerous cancer, kawaski disease and more.