Subject to financing

[Nicolas Ake]

Can someone please explain how subject to works when someone still owes most of their mortgage? 

It seems that any cashflow would have to go to paying the original owner to incentivize them? So let's say $124 a month for 41 years to pay them $65,500? And that 0% interest (original price of house). All while I'm repaying the actual mortgage for 30 years? Am I completely wrong in my understanding of how this would work theoretically? Do you pay the the full price or just on their equity?

Are you able to refinance a subject to deal to put cash on another deal?

(original owner benefits from cashflow from deal one, while I get to use the equity and get the property in the end?) 

I'm having trouble understanding. Please help me clarify. 

Thank you!