to blow the nose with fingers
擤 centers on the action of blowing one's nose using the fingers, a specific physical gesture combining the hand and the nose.
擤 combines 扌 (hand) with 鼻 (nose), directly depicting the action of blowing the nose with the fingers. The historical development is straightforward from these components.
The hand radical 扌 beside the nose 鼻 shows a hand pinching or wiping the nose: blowing the nose with fingers.
For コウ, imagine blowing your nose into a tissue and making a loud 'koh' sound: koh -> コウ, as your hand helps clear your nose.
nose-blowing