At collision energies above 1 eV an insertion mechanism is shown to dominate in the hydrogen exchange reaction. The cone of acceptance for reaction is found to be made up of an inner cone (i.e., for more nearly collinear collisions) where exchange proceeds by abstraction and an outer spherical sector where the mechanism is by insertion. The cross section for reaction, computed by classical trajectories, declines at energies above ca. 1 eV due to a recrossing of the transition state after a collision with an inner hard core. Thus, while the barrier to insertion is higher, this mechanism dominates for such hot H atoms as are currently available from photodissociation. For the H + HD reaction with rotationally cold HD, the cone of acceptance about the D atom is significantly wider.