There are two ways to hold ones breath, one is to take a breath and block the throat, when one do that the muscles in the chest relax and fall down and a pressure builds in the lungs, the poisons, carbon-dioxide and others goes back into the blood stream again and numb the body.
The other way to hold ones breath is to keep the tension in the muscles of the chest without blocking the air ways of the throat, when one do it this way the pressure of the air in the lungs is the same as outside and the body can expel toxins out.
As always relax your head on the spine and use one hand to block alternately the left and the right nostril. Lisen to the bodys need for air and let your body breath as it will. When the session is over just sit and relax and breath as the body will. Always do it in a relaxed maner. Remember to balance your head on the spine so that you facilitate the energy flow and make it easy to break up the blocks between the mental and the body awareness. And take it easy, do not overdo, slowly slowly, let yourself use time to adapt and change.