Open.btc is same as btc, just imagine 1gm of gold you want to trade it in olden days(say 1900's) what you will do, deposit in a bank or trust and get equalent contract and place that contract in market. So if someone agrees to buy it, you will give that contract. The person who hold the contract is the owner of that 1gm gold now. This gives the mobility of gold and security. Hope you understand open.btc and btc now. Thats why they say open.btc is iou(i owe you)