I extracted this from one of a old forum:
Fast updates are a mechanism used to quickly redraw the image. Since it has to sent the entire image quickly.. typically it will be at a lower picture quality.. this is what you see as 'pulsing' in the video as the image gets blurry, and then sharpens up as more detail is sent after the fast update.
Remember, the video codecs we use today operate by sending the differences between frames. So the entire image is not sent all the time.. just part of the image. The changes are then added/whatever to the existing picture.
If you are missing a bunch of information, your system will have a hard time rebuilding the picture.. so it will ask for a full refresh of the picture so it has something clean to work with. This is an example of the FAR-END asking for a fast update request.
However, Fast updates are also normal when the system has to quickly change the entire image.. like when you switch camera sources. So Fast updates are normal.. when they are not, is when a system persistantly keeps asking for Fast updates.
In the case of packet loss, often a codec will request a Fast Update Request (FUR) when it don't have enough info to draw its picture properly.. so it requests a new full image.
A codec will request for FUR if it doesn't believe it has a good image, or info to build the image with, it can as for FUR to get a new image to work with.
Also, a not so ideal network environment that has packet loss will also casue the codec to send FUR, as little as i packet drop will cause FUR.