It ends the session and the rider is to return to the pits ............ those that I have seen (and they are not MotoGP rules) do not make it mandatory that a rider should slow (will check FIM rule book later)
It was once said to me that the reason why you often do not find the words 'must slow' in a rule book is that you cannot define slow in terms of an acceptable decrease in speed.
It does exist in regards to yellow flags and even then is ignored (it is not known as the catchup flag in car racing for no reason - another of my personal peeves as there is not enough attention bought to punish constant flag ignorers)
Edit. From page 49 of the FIM Rulebook -
Regulations and documents: FIM Website
Chequered Black / White Flag
This flag will be waved at the finish line to indicate the finish of race or
practice session.
At a glance, below is the only mention of having to slow
Two yellow flags waved together at the flag marshal post indicate that
there is a hazard wholly or partly blocking the track.
The riders must slow down and be prepared to stop. Overtaking is
forbidden up until the point where the green flag is waved.