Q&A on the Contemplative Practice of the Four Establishments of Mindfulness, 25

Question: Last night starting at 11:30 I sat in meditation on the sofa in half-lotus position. After reciting the Heart Sutra, my mind calmed down, and I began observing the breath, gradually entering a state between sleeping and waking. After about 30 minutes, my body felt comfortable and light, floating. Slowly my body began to vibrate, and the vibrations grew larger and larger... I couldn't control myself anymore; there was no breath, my body was swaying, my eyes wouldn't open, my heartbeat was accelerating and I felt like I was about to die... I knew it in my mind but couldn't control myself... I was very frightened, feeling like I was about to die. I couldn't speak, couldn't cry out, couldn't move, and only with great difficulty did I fall over. It took a long time to regain my calm. 

Answer: When first cultivating meditative concentration (samādhi), after the mind becomes calm and the qi and channels circulate well, there may be a phenomenon of the qi mechanism stirring. This is a good sign of opening up the channels throughout the whole body, and it can regulate the body very well. If your physical condition is very poor, do not enter meditative concentration all at once; take it slowly, regulating the body bit by bit, so as to avoid being unable to adapt to something so sudden. 

At the beginning, entering and emerging from meditative concentration will be involuntary; the body is controlled by the qi mechanism and does not quite obey your own direction. After a long time, it will ease considerably, and you will be in control and able to decide for yourself. When the body's qi mechanism stirs, just relax and don't worry about anything. The emptier the mind, the better; the more untroubled, the better. Just let yourself empty out—be willing to be empty, dare to be empty. Once you are empty, there will be no problem.