I came across this post on reddit today that explains the act very well. This post is broken into two parts. First is just a conversation about the general idea, the second which is after the fold (more) contains a better rundown which is quite clear.
Bob: Hi, insurance company. I’d like to buy some health insurance.
Insurance company: No. You had cancer when you were 3 years old, and the cancer could come back. We’re not selling you health insurance.
Bob: It’s not my fault I got cancer when I was three! Besides, that was years ago!
Insurance company: If we sell insurance to you, we’ll probably lose money, and we’re not doing it.
Bob: But I need insurance more than anyone! My cancer might come back!
Insurance company: We don’t care. We’re not selling you insurance.
Obama: Hey, that’s totally not fair. Bob is right, he does need insurance! Sell Bob some insurance.
Insurance company: If we have to, I guess.
Mary: This is cool. Obama said the insurance company has to sell insurance to anyone who needs it.
Sam: Hey, I have an idea. I’m going to stop paying for health insurance. If I get sick, I can always go buy some insurance then. The insurance company won’t be able to say no, because Obama’s told them they have to sell it to anyone who needs it!
Dave: that’s a great idea! I’m not paying for health insurance either, at least not until I get sick.
Insurance company: Hey! If everyone stops paying for insurance, we’ll go bankrupt!
Obama: Oh come on Sam and Dave, that’s not fair either.
Dave: I don’t care. It saves me money.
Obama: Oh for god’s sake. Sam, Dave, you have to keep paying for health insurance, and not wait until you’re sick. You too, Mary and Bob.
Mary: But I’m broke! I can’t buy insurance! I just don’t have any money.
Obama: Mary, show me your piggy bank. Oh, wow, you really are broke. Ok, tell you what. You still have to buy insurance, but I’ll help you pay 95% of the cost.
Mary: thank you.
Obama: I need an aspirin.
Insurance company: We’re not paying for that aspirin.