Not sure about TAR, but you can achieve something like what you described by taking your clip and splitting it where you want the "normal" speed to start. Then go to the new clip you just made (the first part of the old clip) and increase the speed on that clip by either holding the 'Ctrl' key and sliding the beginning edge to the right, effectively shortening the clip (by speeding it up) or go to the clip properties and set the playback rate to 4.00. Unfortunately, the fastest you can increase it is 4x, by doing it this way. To get the playback rate any faster, you would need to get it to 4x, then render that clip only to .avi, then repeat the process.

If the motion you are describing speeds up in an ever-increasing rate, then I don't know how to do that smoothly.

Hope that made sense.