Pain is always caused by Blood Stasis. In this case Blood is not the red liquid you have in mind, but rather an unseen mixture made of of Original Qi and Nutritious Qi. To make it simple, Blood is made of two specific type of Qi and it is then spread to our organs and tissues thanks to the network of large and smaller meridians. If for any reason this qi does not flow and it stagnates, it creates pain.
In Chinese they say: 通则不痛、痛则不通 (tōngzé bù tòng, tòng zé bùtōng), which means “if it flows, it does’t hurts, if it hurts, it doesn’t flow”.
This is one of the reasons we want the Qi to move: when it does not move and it may accumulate in one part of the body and thus create pain. Body tapping or any movement that creates vibration in the body helps to unblock stagnant Qi.