Were does the air go when you pinch your nostrils and blow?

does a pressure of air build up at the back of the eyes?

[Paderla Anitha, DNB]

Hello and hope you are doing well.

You could be referring to the valsalva maneuver, where air is blown into the cheeks against a closed nostrils, this sometimes helps to open up the 'Eustachian  tubes' which connect the middle ear to the throat, this balances the air pressure in the ears.

Hope this helped and do keep us posted.