Briefly, the repartimiento system saw the Spanish crown sending elites to their colonial territory of Oaxaca to govern the native populations there, without any more extensive imperial presence. These elites wanted to make a little on the side, and the local populations (poor by European standards to begin with, and significantly impoverished since the arrival of those Europeans) were desperate for credit. The Spanish elites fronted them cash or expensive livestock at high interest rates; these debts were to be repaid with cash crops that the elites could take back to Spain for large profits.
The dominant view is that this enterprise was coercive, and in the context of overall Spanish-Indian interaction, it strikes me as naive to assume that the repartimiento system was not characterized by racism, violence and underhanded dealings. Jeremy Baskes, though, emphasizes that the reach of the Spanish Crown was not very strong in this region, and that the individual Spanish elites would have been best served through market exchange, since they didn't have the force to fully dominate the population.
The final sentence of the piece: "Indians accepted repartimientos voluntarily."
Now, it clearly wasn't a euvoluntary exchange. The elites had the power to imprison debtors at will and the Indians were much, much poorer. Given those constraints, though, the Indians weren't actively coerced/enslaved, and were in many cases made better off through access to this foreign capital.