You have to keep blowing out all the to air
ie keep blowing bubbles as your face is turning to the surface and until it bresks the surface.
if you dont water will go up your nose when it tuns sideways
you also need a brief blow as you turn you face back in.
Dive underwater face down and roll over onto your back
water will rush up your nose once you start to turn over
now do it again blowing bubbles out the whole time and you will see that no water can get in.
you dont have to blow from nose the action of blowing from the mouth will seal off the nose and give a small trickle from there