There’s really only one sure way to get contraband through customs… and that’s up one’s arse. I am sure that’s what is being referenced here:
The vague (or not so vague) reference to “passing stones” could also apply to the kidney variety (ouch!) but I am sure we are closer to this:
The classic scene in Pulp Fiction where Christopher Walken passes the gold watch on to Bruce Willis’ character as a young boy… What an awesome movie… What a wonderful cameo! And the fact that Tarantino, as director, has done a cameo in most (all?) of his own movies is a ready topic for discussion.