This is a very effective effect, easily created...
Note: This effect works best with vector images, even better with text.
1. Create the effect for the text/image yo are going to use.
2. Copy and Paste the object 3 times.
3. Do the same for the effect (If hasn't copied already).
4. Now arrange each effect in the timeline so there is 1 frame space between each effect.
5. Now you need to make the copied objects semi-transparent...
For text/vector images:
1. In the properties toolbox open the color selection box. In the top right-hand corner there will be a box labeled 'A' (Alpha).
2. Select the first copied block of text/ vector image and set the Alpha to 75%
3. Select the second copied block of text/ vector image and change the Alpha to 50%
4. Select the third copied block of text/ vector image and set the Alpha to 25%
For non-vector Images (bmp, jpeg etc..):
You will either have to put each copied image in a sprite, and then place a semi-transparent rectangle over it, or import 4 images, each with different Alpha values (100%, 75%, 50%, 25%)
You should be able to create some interesting effects with this 'motion blur' effect.
Here is an example I made with the explosion effect:
More Flash & Swish: 
