This passage speaks for itself. Ling Shu, chapter one：
“持针之道，坚者为宝，正指直刺，无针左右，神在秋毫，属意病者，审视血脉者，刺之无殆。”
“Technique for holding the needle, hold firmly as if holding a precious treasure, the fingers must be securely set for perpendicular insertion, do not needle [to the] left or right, the spirit is at the tip*, the doctor [must] focus the intention on the disease, check the pulse, acupuncture must not endanger the patient.”
* “神在秋毫” is literally, “the spirit is at the autumn down,” or the new fine down feathers with fine tips. Obviously a tender reference to the tip of the needle.
Nei Jing Eastern Medicine 