SDK on your back without breathing out your nose

Sorry for the lengthy title, but I am looking for advice on how to improve my underwater dolphin kick on my back, without expelling all my breath out of my nose in order to keep water out of my nose. In this video, the girl in the beginning is not expelling air out of her nose while kicking on her back. Then later, the guy does expel air out of his nose, but he must go about 10-15 yards underwater. erg.goswim.tv/.../backstroke---underwater-dolphin---size.html I only seem to be able to go 5, maybe 10 yards (with fresh lungs), but I always have to expel air out of my nose or else I get water in the brain. Any advice on how I can not expel water out of my nose to conserve my breath and kick longer underwater? Thanks!