Pl. refer to Duncan's document for falttings shares. Clear explanation..
It means when DRS moves the VM to its new host if that VM had custom shares on it which are very high then DRS flattens the shares to ensure that the moved VM does not consume all the resources of the host it has been migrated to.
Pl. check this explanation with example.
I would recommend to work out by creating resourece pools and vm's with high/low resources . probably a test lab .