[jessriedel]

It just means that in a negative-temp system the number of states with a given energy gets smaller as energy goes up rather than, as for the vast majority of systems (which have positive temperature), getting larger. That means that energy will flow even more quickly to the positive-temp system from the negative-temp system than it would from any positive-temp system, because each unit of energy transfered enlarges the state space of both systems (rather than enlarging one and shrinking the other so as to simply produce a net increase in the number of states, as is the case for most spontaneous energy flows).