Soab;
One of the important things here is to understand, that we will never understand how it works. If understanding everything that goes on around you is vital, this may not be the place for you (or me).
If you have to ask "why", and get an answer, you will probably fail the blood pressure test (as I did). Then you get to take the 24 hour test, like I did (passed this time).
The one they put on me was just an automatic, battery powered blood pressure tester that tested my pressure every 15 minutes, exactly. Next time I will just pay an 18 year old chinese person to wear it.
Smile and be happy in your ignorance. It is working for me so far. Most of the time.
LOL