The responses so far have the definition of port chuffing covered, more of less. During loud bassy scenes, you'll always feel wind if you have your face near the port. It's only when the port starts to add sound of its own -- acting like a trumpet of sorts -- that if becomes a problem.
What happens is that the air entering the port undergoes some compression and then becomes highly turbulent once it exits on the other side. Flaring the port is supposed to significantly reduce this turbulent effect.
