Retaining the needle serves two different purposes. In one, the needle is left in place while the practitioner waits for qi to arrive. In the other, qi has arrived and the needle is left to prolong its effect. The classical texts distinguish the two, and so should the practitioner.
Withdrawal is a technique in its own right. In tonification the needle is withdrawn slowly and the point is pressed closed; in reduction it is withdrawn quickly and the point is left open. Either way, pressing the site after withdrawal prevents bleeding and bruising, and the patient should be observed briefly before leaving.