The issue is that when under high load, the components demand more power than the charger can deliver. The system elects to use battery power to provide the extra, but it can only do this on a limited basis as the battery runs down.
I expect they have changed the behaviour so that rather than shutting down as the battery runs flat, the system scales back to within the available capacity.
The update seems to be only for the Early 2011 MBPs. It's to version 1.69f3.
(The Late 2011 revision MBPs presently ship with this version.)