Default behavior when extending/shortening a transition is to apply equally to both sides of the edit point. To change this behavior, open the precision editor by double clicking on the transition. You can move the edit point, the center little box in the expanded clip view, left or right, this performs a roll edit; or grab the end of the video clips and extend or shorten the clips for a ripple edit.
Option clicking may offer additional editing parameters, as well as opening the video inspector, the video animation editor, adding keyframes in keyframe editor.
Time to play, Happy Editing
Hope this Helps, Greg