  Orbital angular momentum is crucial for the description of several process at
large momentum transfer in the impulse approximation in perturbative QCD. We
review the mechanism of independent scattering which makes violation of the
helicity conservation rule possible at leading order - and without any need to
flip a quark helicity - in exclusive hadron-hadron reactions. We argue that
helicity flip form factors, which violate the factorization assumptions of
conventional approaches and are classified as ``higher-twist," are nevertheless
calculable to leading order in perturbative QCD when information on non-zero
orbital angular momentum is incorporated.
