Depending on ambient temperature and how hard the brakes are being worked, if you don't need quite so much brake-cooling, you can reduce the effective size of the duct, and thereby reduce drag.
A bit of tape does the job, and can be removed quickly in a routine pitstop if track or brake conditions have significantly changed.
EDIT:
Sorry, totally misunderstood your question for a minute there. Yes you are right there is a splitter in there.

But I'd be guessing, apart from the obvious channelling role to disc and caliper, about it's exact purpose.