Tell him to travel safe, then make his knees buckle
If you are traveling safe, one of the most important and most obvious rules is to wear your seat belt and “buckle” up.
Ironically, Joe is telling this person to be safe, and then Joe knocks this person down with a punch. When you get knocked down, the first thing to go out is your legs, and hence your knees have no support and “buckle”.
Somewhat of am unfriendly reminder of why this person should be protecting himself while he moves.
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