I can think of two: The first one is when the opener bids no trump. Then the responder knows that the opener has a "balanced" hand, and 15-17 high card points for 1NT, 20-21 for 2NT, etc.
Another example is when the opener bids 2 clubs (strong), the responder 2 diamonds (waiting), and the opener bids a suit (or no trump). Then the responder knows opener's strength (22+ points) and preferred suit.
In these cases, the responder is "captain," because he knows his own hand, and has a pretty good idea of opener's. The opener, on the other hand, should "dance to his partner's tune" because he can't match his partner's knowledge of the combined holdings.
Are these the main instances, or are there other instances of responders' captaincies that I have missed?