The blue line rises sharply as the intake valve closes and this causes a pile up of air which becomes a positive wave reflected back up the runner and the red line shows that wave arriving at the runner entrance later.
The blue line rises sharply. The intake valve closes. This causes a pile up of air. This air becomes. The runner shows that wave. This was what a positive wave reflected back up. The red line shows that wave. This was what a positive wave reflected back up. This wave is arriving at the runner entrance later.