They can concede the ground but when the players come into their area they can apply pressure better. What part of that do you need explaining?
All of it really. When you press you press with the forwards first then the CMs. The forwards were retreating to full back positions, so if the CMs pushed even as far as the half way line, they'd be literally ahead of the forwards.
The problem was for me was that they were not breaking out when we received the ball, not how deep they defended. The deep-sitting seemed like a clear instruction to me to avoid leaving spaces for City's really fast forward line.