This is a quick little run-through of how FCP X adds transitions and then allows you to modify them. Just enough to get you thirsty for more, I hope.
Click on an edit point to select it. Make certain that the Edit Point is one that has Handles, in order to allow an overlap for the transition to take place.
Press Command-T to insert the default cross dissolve, or else choose Edit > Add Cross Dissolve. A standard one second cross-dissolve will be added at the edit point.
Tip: 1 second is the default length unless there is insufficient handle space for that, in which case the transition will be as long as it can be up to 1 second.
Cross dissolve in.
Select one of the edges of the transition and drag back and forth to modify its length, A tool tip appears indicating how many frames are being added or subtracted as you do.
Place the skimmer in the middle of the transition and it turns into the Roll Tool. This enables you to drag the edit point under the transition back and forth to change the timing.
Rolling the Edit Point.
To make fine adjustments, hover the skimmer over the edit point, so it turns into the Roll tool and press , (comma) or . (period) to roll one frame at a time.
Open the Media Browser (Command-5) and have a look through the Transitions:
Media Browser, Transitions.
Skim over he Thumbnails to see an instant preview of the transitions before selecting one. Choose Dissolves > Fade to Color.
Drag the Fade to Color transition from the Media Browser and place it over the top of the Cross Dissolve you inserted earlier.
When the Cross Dissolve highlights with a white box you can release the new Fade to Color and they will swap (note the duration and position stay the same).
The Cross Dissolve highlighted.
Select the transition in the Project Timeline and open the Inspector (Command-4).
The Inspector Window contains all the editable parameters for the selected transition. In the case of this Fade to Color you can control three areas:
The color picker.
Select the Grey transition in the Project Timeline, hit Delete and it's gone!
Transitions in FCP X offer some really nice options, experiment some more.