This is quite a good discussion of the first breath I think. If you scroll down a little it is under the heading "The First Breaths" in the second column
Basically it is not completely understood but is probably a combination of stimuli such as change of temperature and pressure combined with noise, light, touch etc which stimulates the brain stem to trigger the first breath.
It is unusual but some babies do take their first breath "on the perineum" after their head is born but before their body.
Cutting the cord isn't a trigger of the first breath but the cessation of blood flow through the cord (whether due to clamping and cutting or natural vasoconstriction) does play a role in the change of circulation from the foetal pattern to the neonatal pattern.
Do you mind me asking why you ask "what the medical field say..."? Do you think they/we have it wrong? Is there another theory?