What was it about the death of Jesus that allows God to forgive us?

I've heard many people state that it was the punishment inflicted by the Romans that allows God to forgive us. This just doesn't make any sense to me. How can a man suffer for a short while here on earth in order that a divine being have the right to forgive sin? What is the framework of [substitutionary atonement](http://en.wikipedia.org/wiki/Substitutionary_atonement) that makes Christ's death effective?