How did Maedhros stay alive?

[Galadriel55]

And Morgoth...set his power upon [Hurin], so that he could not move from that place, nor die, until Morgoth should release him.

~The Words of Hurin and Morgoth, COH

This quote supports what Nerwen said. However, even as the greatest Ainu, Morgoth is inferior to Eru, and Eru holds the power to give or take im/mortality. It's not up to Morgoth to decide to make a human immortal until he got what he wants?

Or is it just withholding the death, not making one completely immortal?

I don't get it.