In ancient times, you either walked or took a sedan chair. No matter what kind of exercise, it will drive the air to flow and form wind, such as: wind under your feet, dust all the way, etc. It is such a delight to have friends coming so far. When friends stop, the wind driven by friends' walking stops. To make a metaphor, catching the wind formed by a friend's walking is equivalent to catching a friend. This is the courtesy that a host should have to his friends.