Bahya Kumbaka refers to the time after exhalation and before inhalation. It is the time when the lungs have little or no air.
In advanced stages the lungs are completely emptied but in the earlier stages it is much more fruitful to allow a little air to remain in the lungs.
During Bāhya kumbhaka a lock can be performed. It is called Uddīyana Bandha. The head is dropped and the lock of Jālandhara Bandha are also performed during Uddīyana Bandha.
"There is a deep change in the mind waves when the lungs are completely empty. It is a time of deep detachment"