  We investigate the spin dynamics and the conservation of helicity in the
first order $S-$matrix of a Dirac particle in any static magnetic field. We
express the dynamical quantities using a coordinate system defined by the three
mutually orthogonal vectors; the total momentum
$\mathbf{k}=\mathbf{p_f}+\mathbf{p_i}$, the momentum transfer
$\mathbf{q}=\mathbf{p_f-p_i}$, and $\mathbf{l}=\mathbf{k\times q}$. We show
that this leads to an alternative symmetric description of the conservation of
helicity in a static magnetic field at first order. In particular, we show that
helicity conservation in the transition can be viewed as the invariance of the
component of the spin along $\mathbf{k}$, and the flipping of its component
along $\mathbf{q}$, just as what happens to the momentum vector of a ball
bouncing off a wall. We also derive a "plug and play" formula for the
transition matrix element where the only reference to the specific field
configuration, and the incident and outgoing momenta is through the kinematical
factors multiplying a general matrix element that is independent of the
specific vector potential present.
