The end does sit on top without being fixed so there is a possibility that it will lift up and down as in the video. 

If you have too much movement you may need to adjust your awning settings. 

Once you extend the awning out and then fit the AFK, you then need to wind it back slightly before tightening the clamps. 

Have a look at the video below. Just before 3.00 minutes into the video, it shows how once they have fitted the AFK they wind the awning back in to make the fabric tight before they then tighten the clamps. 
Click here for video

Once you have done that, the AFK should fit better but you will always be able to move the plastic up and down as this is required to be able to remove the AFK from the awning.

For further stability, you can also purchase a curved center rafter and tie-down kit.