Penal substitution is really a legalistic way of looking at things. It basically is like a debtor's prison, and you have a massive amount of debt that you can never hope to pay off. The person who you owe it to is God. God is angry because you burned down His house (as an example), and so He had you thrown in prison since you can't ever hope to pay off your debt to Him for His house. Despite the fact that He has 1,000,000 other houses and can restore this one in a moment's notice, He is furious at you and your inability to pay your debt to Him, on the principal of the thing. However, he comes up with a brilliant idea. The only person who can possibly give Him as much money as His house was worth, is Himself. So, He sends His Son to the bank to withdraw the massive sum of money, and His Son delivers it to Him. Now, since someone (Him) has paid off your debt, he isn't mad any more and everything will be all right. That is penal substitution, you owe a debt to God that only God could pay, and he refused to forgive the debt until He sent His Son to be brutally tortured and executed in the most inhumane way possible, so cruel that the Romans forbid its use on Roman citizens. Does that make much sense?
Rather, Orthodox see it more like this: The natural consequence of sin is separation from God that results in a bodily death. This is much like the natural consequence of you throwing a bowling ball straight up into the air above your head, is that it will come crashing down on you (this is NOT like the natural consequence of refusing to pay your rent is your landlord kicking you out of your home). A bowling ball landing on your head will kill you. However, God cannot die. Therefore, to save you from the troubles you have caused yourself, He sends His Son to take your place, knowing that His Son will be unable to be destroyed by the bowling ball.
My analogy has a lot of problems in it, but that is the gist of Orthodox salvation theory.
If anyone has complaints with how I have portrayed something or with my apparent understanding of something, please let me know.