Roughly speaking, the idea of "modes of responsibility" is that they are an intermediate step between the first principle of moral action - that good should be done and evil avoided - and specific moral norms describing certain actions as right and others as wrong. Each "mode of responsibility" represents a "way" or "manner" of acting that is ordered towards a particular, more precise expression of a human good. It stands between the idea of "good" in general and one of a range of specific goods in particular.
A brief statement of the idea of "modes of responsibility" can be found here. A presentation of the idea of "modes of responsibility" in the words of Germain Grisez's The Way of the Lord Jesus can be found here.
I wonder whether this idea of "modes of responsibility" as an intermediate step between the first moral principle and specific moral norms can shed some light on a way in which we might understand Pope Benedict's remarks about condoms (my emphasis added to a quotation taken from the CTS website)?
There may be a basis in the case of some individuals, as perhaps when a male prostitute uses a condom, where this can be a first step in the direction of a moralization, a first assumption of responsibility, on the way toward recovering an awareness that not everything is allowed and that one cannot do whatever one wants. But it is not really the way to deal with the evil of HIV infection. That can really lie only in a humanization of sexuality.
Are you saying, then, that the Catholic Church is actually not opposed in principle to the use of condoms?
She of course does not regard it as a real or moral solution, but, in this or that case, there can be nonetheless, in the intention of reducing the risk of infection, a first step in a movement toward a different way, a more human way, of living sexuality.