So Watford was defending with timings to control (no pressure) and timings to do pressure (normally when City does a pass back, lateral pass or vertical pass in the center). This means the defensive structure/organization by Watford needs coordination for timings to control (just keeping the positions) or to do pressure, timings to go down (to control the space in depth) or to go up and reduce the distance between the back 4 and the midfielders.
Watford was playing in zonal marking – medium/low structure there is not to much space behind the back 4 but City needs this space to create situations to score so how they create that space?
So City “invites” Watford to do pressure when they do a lateral pass, a pass back or vertical pass in the center so the defensive structure by Watford needs to give some steps forward and the space behind the back 4 is bigger than before. So this is the right time for City to play in depth. So defenders are coming up to do pressure and it is the right time for some player from City to run in depth because opponents are runing in different direction. When Watford gives steps forward City knows there is space behind the back 4, it is just to create the right situation/moment to play there.