A hold harmless agreement is contractual, written into the subcontract rather than the insurance policy. The sub agrees to indemnify and hold the GC harmless from claims arising out of the sub's work, shifting financial responsibility by contract.
Hold harmless language and additional insured status work as a pair: the contract creates the obligation, and the insurance backs it with dollars. Enforceability varies by state and by how the clause is written, so legal review matters.