Help with "Shorting a Second" in a "Subject To" Transaction

[Seti Gershberg]

I recently saw a presentation on Subject To transactions and the presenter suggested an advanced concept that involved buying a property that was underwater and had two mortgages. He mentioned you could short the second and create substantial equity. The presentation was basic so he did not go into detail on the concept. I wonder if an advanced investor would be able to explain this concept in further detail?

Many thanks.