Commonly used arm positions can overestimate blood pressure readings: study

Maybe someone could take the time to explain how it actually works. Somehow I missed it in my engineering education.

You have a bladder that goes around someone's arm, and it is inflated. It slowly deflates, and somehow this tells you the pressure in the blood vessels inside the arm.

But that raises some questions:

1) Your arm isn't just blood vessel, most of it is bone and muscle. And fat.

2) How does the inflation help? What about the deflation?

3) What is on the other end of the device?

The cuff inflates to constrict the brachial artery until it is completely closed (i.e. the cuff detects no pulse, i.e. the external pressure from the cuff exceeds the maximum systolic blood pressure). Then it deflates until it detects a pulse as blood once again is able to force through the cuff's constriction (this threshold tells it the systolic blood pressure). It continues deflating until once again it detects no pulse (i.e. the minimum diastolic blood pressure exceeds the pressure exerted by the cuff).