Joe Orenstein - When talking about the motion of charge current, you can...

When talking about the motion of charge current, you can think of the electrons as acting like a swarm of bees moving in one direction. Within that swarm, individual bees might be colliding, but momentum is conserved with each collision so that the total motion of the swarm is unaffected. When talking about the motion of spin current, the electrons act more like a swarm of honey bees and a swarm of bumble bees trying to move through one another. As the bees in these two populations collide, there is an exchange of momentum that slows the relative motion of each. Eventually both swarms may move in a single direction, but the overall effect has been a drag on their collective motion.