What is the mechanism behind inhaling air into one's belly to 'inflate' it?

[Platypus0]

I know most people are able to inhale air to expand their bellies (belly breathing), and some are able to make their bellies become larger by breathing in a certain pattern which imitates a pump to kind of slowly push out their abdominal walls with each breath.

May I know the mechanism behind this? (Pressure, volume in cavities, diaphragm movements)

What happens when a person retains this pushed out abdominal wall for a prolonged period? Will it cause hypoventilation (since the person is unable to breathe properly)?

Is this action similar to a Valsava manuever? Will this cause one's heart rate to speed up?