subshock. The resulting behavior is fairly easily described if the speed of the CRs is large compared to the bulk motions relative to the shock and if we can approximate the distribution function, $f(p)$, as almost isotropic in both the upstream and downstream frames of reference. In that case, for example, the flux of CRs crossing the subshock from upstream in a momentum range dp is approximately $\pi v p^2dp f(p)$,where $v$ is the particle speed. On the other hand, the CRs downstream are advected away at the bulk flow speed of the postshock plasma, which we can call $u_2$. The flux there is $4\pi u_2 p^2dp f(p)$. In a steady state the ratio of these two fluxes determines the probability that a particle entering the shock from upstream will escape downstream at the same momentum, ${\sf P}_e$. Since the steady downstream distribution function must be uniform and also cannot change across the subshock (the CRs ignore the subshock, remember), this gives ${\sf P}_e \approx 4u_2/v << 1$. Thus, under these assumptions most of the CRs are temporarily trapped between the upstream and the downstream flows. Also, by construction they are not stopped within the subshock itself, so that they 