  Generalized Parton Distributions (GPDs) provide information on the
distribution of quarks in impact paarmeter space. For transversely polarized
nucleons, these impact parameter distributions are transversely distorted and
this deviation from axial symmetry leads on average to a net transverse force
from the spectators on the active quark in a DIS experiment. %The strength of
that force can be related to twist-3 PDFs. This force when acting along the
whole trajectory of the active quark leads to a transverse single-spin
asymmetries. For a longitudinally polarized nucleon target, the transverse
force implies a torque acting on the quark Orbital Angular Momentum (OAM). The
resulting change in OAM as the quark leaves the target equals the difference
between the Jaffe-Manohar and Ji OAMs.
