Spin drift velocity?

I am currently reading this Phys Rev paper by H C Torrey. In this paper, he derives the Bloch equations with an additional diffusion term. He says that the current density is given by $$\mathbf j_{\pm} = n_{\pm} \mathbf V_{\pm} - D \nabla n_{\pm},$$ where $n$ is the number of spins(up/down).

Now the second term is clear, as this one comes from Fick's first law, but the first one is strange. He claims that $\mathbf V_{\pm}$ is the velocity of positively(negatively) oriented spins (he restricts himself to spin 1/2). Does anybody know what this drift velocity could mean?

• Could you send the paper somehow? I am unable to access it. – Yuzuriha Inori Mar 10 '18 at 7:43