Trust does not only apply when the team has possession of the ball. It is also extremely important when defending. You have to trust what players behind you are communicating. They see what is developing behind you, so they may instruct you to do something that you do not agree with at the moment. Your entire defensive system depends on the efficiency of each player performing his/her function.
Forwards not being lazy and squeezing the outside backs/ disrupting the center backs, outside midfielders tracking back and tucking inside, Goalkeeper staying connected.
It is important to recognize the importance of the goalkeeper on both sides of the ball. When in possession, teams like Barcelona push their fullbacks high. The centerbacks open up and the holding midfield drops deep as an outlet. The goalkeeper is used to swing the ball if the holding mid is covered.
The team must trust his ability on the ball and his ability to distribute efficiently. When not in possession, Barcelona tries to win the ball back very high up the pitch. The Goalkeeper has to stay connected to the team, so that compactness is maintained. Anything over the top must be met by a goalkeeper with an excellent starting position.
The trust factor also comes from a coach, who gives his players the freedom to play with imagination within the team structure. This is the ultimate compliment to a player, but it comes with a very high level of responsibility. Barcelona leaves three sometimes behind the ball when attacking.
Some think this is very risky, as they are vulnerable to counter-attacks. The coach and the players trust each other. The coach trusts the 1v1 defending ability of his players. His players trust in the effectiveness of the system and they put 100% into it.
All the great teams share these elements of trust. It is what separates the good teams from the great teams.