Deqi is the response that indicates the needle has engaged the qi. The patient reports soreness, distension, numbness or heaviness around the point, sometimes spreading along the channel. The practitioner feels the needle tighten, as though it were being held.
Classical texts treat the arrival of qi as the treatment itself, not as a preliminary. The Nan Jing says to insert when qi arrives and withdraw when it has gone. Modern research has not settled what the sensation corresponds to physiologically, but clinically its presence correlates with response, and its absence is a reason to adjust the point, the depth or the manipulation rather than to carry on regardless.