Say I get $90,000 to go buy a house, I spend the $90,000 and have 1 door. I get a tenant in it and they make a $90,000 down payment. I then go buy another house and get another tenant in it and they make a $90,000 down payment. I now have two houses and still have the same amount of money I started with, I can continue this cycle and as long as property appreciates I don't just get the property appreciation from one house I get it from all of the houses so (% appreciation * property value * number of properties).
Say it is a $100,000 property and it appreciates 3%. If I had just bought one house and put the money in an interest yielding savings account I would get some low % return. If I instead chained it to acquire 10 houses I would get $30,000 return, which is 30%. But if the value of the house goes down it works against me!
Obviously this ignores transaction costs, interest costs etc.