It’s like assuming someone’s mortgage without the formal assumption package/lender approval. You basically start paying the previous owner’s liability “subject to” the terms and conditions of their in-place finanacing. One day they are paying the loan and the next you are.
Caution: This DOES violate the lender’s due on sale clause (DOSC) and makes the loan subject to immediate demand to be fully due and payable. Even though this doesn’t happen often, you need to know that it legally can happen.