Why is the three-nucleon force so odd?

By considering a class of diagrams which has been overlooked also in the most recent literature on three-body forces, we extract a new contribution to the three-nucleon interaction which specifically acts on the triplet odd states of the two nucleon subsystem. In the static approximation, this 3N-force contribution is fixed by the underlying 2N interaction, so in principle there are no free parameters to adjust. The 2N amplitude however enters in the 3NF diagram in a form which cannot be directly accessed or constrained by NN phase-shift analysis. We conclude that this new 3N-force contribution provides a mechanism which implies that the presence of the third nucleon modifies the p-wave (and possibly the f-wave) components of the 2N subsystem in the triplet-isotriplet channels.