569 postscript
I haven't tested it, but the current value the loop is in gets added to the stack and show reads the first value in the stack. (I think)
thus first 5 got added to the stack, show reads it (might remove it from the stack) and prints it. loop begins again, 6 gets added to the stack...(again, haven't tested it and I have never done anything with it so....yea...)