To insert the soft needle with the round shaped tip, the pressure applied from up to down with one's fingers has to be transmitted to the tip of the needle correctly. When the vector indicating the direction of pressure applied to the needle is in alignment with the tip of the needle, one can minimize the degree of damage towards the human body. The gap between the vector of pressure applied to the needle and the tip of the needle means the unstability of the needling, namely it makes the numbers of cells injured by the needle increase.
The basic training to insert a soft needle without making it bend would be the most appropriate practice to know such a vector. In other words, if one uses a hard needle, even though the vector is not in alignment with the needle tip and makes the needle unstable, it can penetrates the body easily without being bend. This is so called "painful needle", the unstable needling, dispersing the pressure applied to the needle. Those who are used to hard needles are prone to get stuck in such needling. Just apply the pressure towards only one point, the tip of the needle. When you can do this precisely, you are able to insert a needle. This is called inserting ability.
* This article is translated from Japanese Shakuju Association blog.