Question

How to understand air pressure against a hand pump?

Answer 1

hey mate ur ans is
when u pump the hand it create a pressure inside it which pull the water upside continiously..

Answer 2

you can understand it by the method of straw you use to drink cold drink .while drinking we suck the air and because of it the air comes out and a vacuum is created in the straw to eliminate this vacuum the cold drink from the bottle enters in the straw and we can drink it .
same happens with the head pump
I hope this will help you