How does a full subtractor work?

[mattbonner]

I understand how a full-adder functions, it's simple enough. However, I'm having a hard time grasping how a full-subtractor works...

I found this diagram

http://www.art-sci.udel.edu/ghw/phys245/05S/assignments/answers/images/13-35sub.gif

but I'm not understanding how the Borrow-in's and the Borrow-out's work...

 

[Jonas101]

I have the same problem , i got this project i need to do with full subtractor but i can't figure it out how should i do it. Its a subtractor for 2 bit binary numbers.