the tailswitch has nothing to do with mode selection and there is no physical switch in the head to switch modes but rather is done because the ring contact on the board is no longer making contact. Yes I know technically that is the switch...
What I mean is that he could have mixed the head and the tailcap. So when both are tightened the light runs properly on high, but when he loosens the tailcap for lower modes, the light turns off.