Why Does Airflow Decrease When Blowing Directly Into a Pipe?

[swerider]

if you use your mouth to blow air through a pipe (an A4 page rolled into a pipe for simplicity) with your mouth against the pipe, the outlet flow is much less compared to when you blow through the pipe with your mouth away from it. In terms of fluid mechanics, why does this happen?

 

[minger]

Because if you have you lips against the paper, then the only air that can come out of the paper. Basically, in order for air to go out, air must come in. If your lips are against the paper, you're stopping air from coming in.