In the Hatha Yoga Pradipika it says, Uddiyana bandha is called the rising or flying bandha because through its practice, the great bird (shakti) flies upward with ease (III.56), and Pulling the abdomen back in and making the navel rise is uddiyana bandha. It is the lion which conquers the elephant, death (III.57). Of the three major bandhas (the others being jalandhara bandha and moola bandha), it's the one that often feels the most accessible. The inward lift of the belly seems to focus the fiery strength of the agni, and unleashes a usually untapped storehouse of power. Because it can only be practiced on the exhale it also seems to create space – space to be still, space to feel uncertain, space to embrace emptiness.
This controlled emptiness is called bahya kumbhaka. As BKS Iyengar writes in Light on Pranayama, Bahya kumbhaka is the state in which the yogi surrenders his very self, in the form of his breath, to the Lord and merges with the Universal Breath. It is the noblest form of surrender, as the yogi's identity is totally merged with the Lord.