You can change the alignment of a transition placed between two clips in either the Timeline panel or the Effect Controls panel. A transition need not be centered or strictly aligned to the cut. You can drag the transition to reposition it over the cut as desired.
Align a transition in the Timeline panel
Align a transition using the Effect Controls panel
in
the Effect Controls panel. If necessary, widen the panel to make this
button visible and active.
appears;
then drag the transition as desired. For finer control, magnify
the time ruler.To place all of the transition in the clip preceding the edit point, drag the transition to the left to align its end to the edit point.
To place all of the transition in the clip following the edit point, drag the transition to the right to align its beginning to the edit point.
To place unequal portions of the transition in each clip, drag the transition slightly left or right. For finer control, zoom in on the time ruler.

You can also choose an option from the Alignment
pop‑up menu in the Effect Controls panel. Custom Start appears as
an option in the Alignment field only when you drag the transition
to a custom location over the cut.