I don't know how it would be done, but I can only see two scenarios. One is to charge the insured the full, unsubsidized premium, and give the tax credit in 2015. The other is to pay the subsidy to the insurance company throughout the year.

In the former situation, some people just cannot afford it.

In the latter situation, you can have some fraud (by the insurance companies).

I guess they could offer both options, and if you choose the latter, you need to apply through the government, so they can verify that you exist, etc.