Mannitol is an osmotically active solute that causes fluid shifts between compartments. Mannitol administration will result in a rise in plasma osmolality which will pull fluid from the ICF to the ECF and thus increase plasma volume. If you were to infuse normal saline into the vascular compartment, only ~1/3 would stay there. I suppose the theory with mannitol use is that, since it doesn't cross membranes, you get more fluid movement and thus more rapid volume resuscitation.
I haven't ever heard much about mannitol in shock so that's a pretty cursory explanation of my understanding of the theory. In real life, I feel like you would risk cellular shrinkage and something akin to central pontine myelinolysis could occur (similar to what happens with rapid infusion of 3% Saline), but I'm not sure on that one.
Hope that helps.