Even though the subject about holding the breath was explained by Nomad and Laura, I just wanted to add that my teacher's advice was exactly the same. Not that I value his opinion more, but it was still interesting to hear his take on this. A few minutes ago I ended our phone conversation where explained the following:
After the inhaling the holding of the breath is achieved by closing the windpipe (glottis) and keeping energy/suspension in the diaphragm. The keeping of the energy in the diaphragm should not be done by tensing any muscles, inner or outer. It's more like a delicate inner feeling, since the diaphragm is to a high degree an nonvolitional "muscle". He described his own sensation when doing the holding as: "Well, I don't really actively think of closing the throat, the closure happens as a byproduct from keeping the energy in the diaphragm [keeping the diaphragm low]. I don't tense any muscles at all. The whole process shouldn't feel arduous. If it does, you are doing something wrong." Touché
He continued explaining that the theory goes that one can't absorb and utilize the "prana" effectively if there is tension in the muscles while holding the breath. Only when this is done in a completely relaxed manner can one get the full benefit of the procedure. He said that he has seen e.g. many expert yogis who have been trained in these breathing techniques, but who do the holding of the breath (kumbhak) incorrectly - keeping tension in chest and emergency breathing muscles.
Hope this helps and clarifies more than confuses :)
After the inhaling the holding of the breath is achieved by closing the windpipe (glottis) and keeping energy/suspension in the diaphragm. The keeping of the energy in the diaphragm should not be done by tensing any muscles, inner or outer. It's more like a delicate inner feeling, since the diaphragm is to a high degree an nonvolitional "muscle". He described his own sensation when doing the holding as: "Well, I don't really actively think of closing the throat, the closure happens as a byproduct from keeping the energy in the diaphragm [keeping the diaphragm low]. I don't tense any muscles at all. The whole process shouldn't feel arduous. If it does, you are doing something wrong." Touché

He continued explaining that the theory goes that one can't absorb and utilize the "prana" effectively if there is tension in the muscles while holding the breath. Only when this is done in a completely relaxed manner can one get the full benefit of the procedure. He said that he has seen e.g. many expert yogis who have been trained in these breathing techniques, but who do the holding of the breath (kumbhak) incorrectly - keeping tension in chest and emergency breathing muscles.
Hope this helps and clarifies more than confuses :)