Heart Rates and Blood Pressure

When physicians measure blood pressure, they look for your systole and the diastole in order to get an accurate reading. After using a sphygmomanometer to briefly cut off blood supply to the arm, the doctor lets the air out of the sphygmomanometer slowly. The time when he first hears your heart beat is the systole, and the time when he cannot hear the beating anymore is the diastole.

As I mentioned earlier in this post, a sphygmomanometer is often used to measure blood pressure because a person can hear the systolic and the diastolic pressures. They can figure out the maximum pressure achieved when ventricular contraction occurs during systolic pressure as well as the lowest pressure during ventricle relaxation which is called diastolic pressure.
To use a sphygmomanometer, wrap the cuff around your arm, just above the elbow with the two tubes on the inside of the elbow so that they aren't tangled up beneath the arm. Then, use the inflation pump to pump air into the cuff so that it reads about 150 on the aneroid manometer gauge, briefly cutting off blob flow to the arm. Next, slowly let air out of the cuff, and using a stethoscope, listen for when you can hear the blood rush back into the arm, and for when the sound of blood stops as the pressure equalizes in the arm. Make sure to not have the cuff on the arm while inflated for very long periods of time, as that could damage the arm.
A person can measure heart rate in many different ways there than using a sphygmomanometer. One of these ways is to use a stethoscope on your chest to hear your heart beat. Another way is to put two fingers (not your thumb) under the side of the chin on your  carotid artery or on the inside of the wrist on your radial artery, and count how many beats you feel in a minute. Heart rate can be determined this way because it is easy to calculate the amount of heart beats per minute.

The reason that people aren't supposed to measure their pulse with their thumb is because you can measure your pulse with your thumb the same way you can measure your pulse with on your wrist and on your neck. Since people can feel a pulse from the thumb, it would interfere with finding the pulse on your neck or wrist.