Act transitions ease your reader from one act to another. Think of them like doorways, one-way doorways to be exact. The transition is the doorway, not a room of its own. There is no returning once you’ve past from one room into the next. You thrust your characters forward, in the eventual inevitability. Once into the next room, the confrontation can take place.
Through door (transition) number one
Here you go from Act I to act II. The struggle that started in Act I continues in the middle section of the book. The key question is: can my main character walk away from the plot right now and pretend nothing happened? If yes, you haven’t really gone through a door yet. There is a difference between an initial disturbance (the inciting incident). The doorway of no return, crosses a threshold.
Through door (transition) number two
Now you move from act II to act III. Thus, the second doorway of no return must send main character towards that knock-out ending. Something has to happen that sets up the final confrontation. Usually this is a major clue or piece of information, a setback or crisis that demands action.