At this stage in your pranayama practice you should start to practise outer retention. That is, it is time to supplement inhalation, exhalation and inner retention (antar kumbhaka) with retention of the breathing process when the lungs are deflated as much as is possible. In Sanskrit, this stage is known as bahir kumbhaka, or sometimes bahiranga kumbhaka.
Bahir kumbhaka is a very important part of pranayama practices and will bestow many benefits. However, as we have already emphasized, you must slowly develop the capacity to retain the breath externally. Don’t try to hold the breath for too long in the beginning, even if at first it may seem easy. This applies particularly to outer retention, for this is a function that the lungs and associated nerves are rarely required to carry out. Each of us has held his or her breath many times, but almost invariably it is with an initial intake of air beforehand. That is, we generally take a deep inhalation before holding the breath. This applies in many sports, such as swimming. We do not normally empty the lungs immediately prior to holding the breath, but fill them to maximum capacity. Thus people are more accustomed to inner retention.