Suppose that you want to apply that effect to track x.
create a fx channel track (fx1)
Select track x and now click "e" to enter in the track eq,slot,effect ecc..
Select the slot 7 on the right ,(out bus) and click left here and select fx1.
(use of 7 and 8 because they are stereo!!)
Turn the slot on and set it volume to 0,00 db.
Close that window.
Now select channel fx that you have created, and after click "e" to enter in settings.
in the slot 7 activate the delay or reverb effect and put it at a standart level.
Close this window.
Select the fx1channel and click where you see a little + on the bottom left of the track.
Another track will appear.
Where you see the word "mute" select pan left right.
Now activate for this track the "r" read automation.
the line of that track will become green.
Take the pencil and here draw a continued "S" that will be your transition from right to left channel.
Click play and listen.