This passage speaks for itself. Ling Shu, chapter one：
“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.