So first what we do know ...
1. Brutalities are activated in the Winning round as a last hit
2. Brutalities thus far are modified special moves or throws
3. There are specific requirements that need to be met to perform them.
My speculation is that once the requirements are met, only need to perform the special move or throw + the meter burn button.
During the stream the brutalities were coming out like standard specials/throws, and there did not appear to be any special inputs before the moves came out.
The other possibility is that some brutalities will be activated by performing a specific combo ala classic brutalities, however if requirements are met, and the combo defeats the opponent, the brutality will come out.
Talk amongst yourselves