T1, T2 and T3 are just any points on the timeline where you decide to add a camera.
If you add a "jump" shot (scissors icon with red down arrow) at say, the 1 sec mark then add a "Move" (arrow icon with green down arrow) shot at the 5 sec you will see it takes 4 seconds for the camera to transition from one to the other. If you drag the "move" shot back to the 2 sec mark on the timeline it will change faster.
Many thanks for the explanation, much appreciated !
I got it now