Since the inception of Freeview, the BBC has always reserved capacity (a proportion of the total bits) for BBC One. This has been done to allow the National and English regional variations of the channel to be inserted in the nation or region.
Whilst this is an effective method of creating the regions, it does mean that any unused bits on the channel (when the picture is stationary, for example) are simply wasted.
Statistical Multiplexing allows all the channels on the multiplex to share all the bits, releasing capacity for other services to improve their picture quality whilst still providing sufficient data capacity to ensure the best possible picture quality on the channel.
The BBC told UK Free TV, that after switchover, they "I can confirm that BBC One is now statistically multiplexed in both pre-switchover and post-switchover areas of the UK."
So what does this mean for the BBC channels bitrates after the retune?
The BBC says that "the decision on what bitrate to allocate to each service at any one instant is taken by the statistical multiplexer based on the instantaneous need of each service.
We don't set a mean bitrate, just an upper and lower limit and a relative service priority. So I'm afraid we can't currently say what the mean bitrates for each service will be (and the mean bitrate is not a good determinant of picture quality anyway)
We have carried out extensive tests with members of the public to ensure that the picture quality following the retune is comparable with that prior to the retune."