oops sorry - I probably misunderstood - koala's post probably is what you want to do? - speed certain sections of a clip up and leave the rest as normal?
If this is the case you need to split the clip into sections - cut it where you want to speed the clip up and again where you want it to go back to normal speed.
To speed (or slow) a clip once cut, drag the end handles of the clip along the timeline further (to the right) to slow it down or drag it to the left to speed it up - in effect what you are doing is changing the duration of the content of the clip.
There are any number of
video tutorials on Youtube