Select the clip. Hit F1 to open up the Propertied Tab. Under the timing section at the bottom change the Time Remap to Variable Speed. Hit the Command and 8 Keys to open up the Keyframe Editor. Go to where you want the speed change to start. Create a Retime Value keyframe. Go to where you want it to return to normal speed and create another Retime Value Keyframe. Grab the second keyframe that you created and the last keyframe which you did not create that is created by default and move them to the left. That will speed up the section you want to speed up.
If you want to do it with the Set Speed Behavior check out this tutorial on creating an exploding wall.
Hope this helps and best of luck.
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