I think that last part might be talking about ‘relative population surplus’ between regions/nations. Migrations are related to the relative pop surplus in relation to regions/nations. The incentive to migrate is inherent to this relation of relativity between regions/nations. The reserve army is relatively larger or smaller in relation to other regions/nations, and that incentivizes migration as a displacement force within those populations with a larger reserve army of labor.

I think about the fracking boom in the USA. Lots of labor force migrated (often temporarily) often from areas with higher relative reserve army to very rural unpopulated areas with almost zero reserve army causing lots of housing issues locally etc. Once labor is forced into an exchange-value relation, it is incentivized to migrate to areas of low reserve army where the exchange-value is greater.