FCP World details an alternate workflow for speed-changing clips in a timeline without that irritating rippling effect that can screw up synch if you aren’t careful. In short, the tip works uses Match Frame to create a temporary master clip in the Viewer. This is a nice workflow, because you don’t have to change your master clip back to 100% speed after you drop your changes onto the timeline. And with a little practice, it’s faster than the “Opt-lock-track” method.

Here’s the good stuff:
1. Select the clip and place the playhead at the beginning of the clip you want to speed adjust.
2. Solo the Auto Select button for the Clip you want to speed adjust. Note: this is for matchframing purposes.
3. Remove In and/or Out Points on the Timeline by pressing OPT + X.
4. Type CMD + OPT + F to load the temp clip into the Viewer. You are essentially performing a matchframe.
5. Press CMD + J to launch the Speed Dialog box.
6. Press Shift + I to get the playhead onto the In Point.
7. Press F11 to Matchframe the speed affected clip back into the Timeline.

More details and discourse on this subject at FCP World.