To start a transition attack, first of all they have to take the ball from opponent. And to do that, they need intensity in the space which also the ball is there too.
For example, in this situation, Arsenal play against Liverpool in Premier League home match. Minute is 18:23. Arsenal are in set defence. In Liverpool, right stopper has the ball.
When the ball is going to opponent’s right back from right stopper, opponent’s right centre midfielder number 14 started to close to midfield from half-space. Centre forward number 20 started to run to Arsenal defense line’s back and with this run, he pulled Arsenal’s left defensive midfielder to himself.
Then right back passes to right centre midfielder and right centre midfielder passes to right winger. In the last picture, as we see, there are a lot of Arsenal players around Liverpool right winger. In this position, right winger tries to go forward with the ball and when he loses the ball, because there are a lot of Arsenal players around him, Arsenal players can take the ball with numerical superiority and start a transition attack.
So now they can start a transition attack. When one of Arsenal defensive midfielders takes the ball, opponent’s midfielders have to press defensive midfielders quickly or defensive midfielders start the transition attack. And to press quickly, already opponent midfielders have to take position closer to defensive midfielders before opponent loses the ball. Or pressing to Arsenal defensive midfielders only work for Arsenal because Arsenal defensive midfielders will have enough time and space before opponent midfielders cover them. And because opponent midfielders try to press, Arsenal defensive midfielders will have to play more quickly. And they can play quickly because they have enough space and time and because of that, Arsenal can start a transition attack more quickly and comfortably. For example, in our situation, Liverpool didn’t press to Arsenal defensive midfielders when they lost the ball and let Arsenal start a transition attack. If they would try to press, because Liverpool midfielders were not close to Arsenal players in set attack, Arsenal players could create a transition attack quickly and comfortably.
After one of the defensive midfielders takes the ball, wingers run to opponent’s defense line’s behind and centre forward and offensive midfielder take position between midfield and forward lines. Centre forward and offensive midfielder make connections between defensive midfielders and wingers. They can take the ball free if wingers are closer to opponent’s defense line. Because when wingers try to escape from opponent’s defense line’s behind, defense line focuses on them and goes back. And when defense line goes back, automatically the distance between opponent’s defense and midfield lines is more and Arsenal centre forward and offensive midfielder find more space.
Also if wingers use half-space to escape opponent defense line’s behind, they can pull opponent stoppers easily. Because when they are in half-spaces, they are threats for both opponent’s stoppers and full backs.
But also I have to say that, if wingers are not closer to opponent’s defense line when they start a transition attack, like in our situation, because they need time to reach opponent’s defense line, Arsenal can’t create a transition attack directly as they want. And also this time because wingers don’t keep busy opponent’s defense line, defense line can focus on covering centre forward and offensive midfielder.