The Ainur and their physical forms

[Boromir88]

A wonderful answer Lal, there's one part of the quote that also catches my attention...

the chains are the cunning of weak and tame minds which have power to resist energy

When we think of actual, physical chains, somebody literally "chained" up to the wall, yes chains do resist energy. The chained person can, rip, tear, thrash, wiggle, all they want, if the chains are strong enough it will "resist" that energy and hold that person.

Chains do not have to be purely physical, they can take many forms, and the chaining of Melkor's spirit would have needed something very different to the metal chain we would normally think of

So then whatever form of a chain that held Melkor, it was able to resist his power.