  In the rest frame of a many-body system, used in the calculation of its
static and scattering properties, the center of mass of a two-body subsystem is
allowed to drift. We show, in a model independent way, that drift corrections
to the nucleon-nucleon potential are relatively large and arise from both one-
and two-pion exchange processes. As far as chiral symmetry is concerned,
corrections to these processes begin respectively at $\cO(q^2)$ and $\cO(q^4)$.
The two-pion exchange interaction also yields a new spin structure, that
promotes the presence of $P$ waves in trinuclei and is associated with profile
functions which do not coincide with neither central nor spin-orbit ones. In
principle, the new spin terms should be smaller than the $\cO(q^3)$ spin-orbit
components. However, in the isospin even channel, a large contribution reverts
this expectation and gives rise to the prediction of important drift effects.
