Whether you are aware of it or not, you pronate after the release of all your pitches (if you actually do ever pitch real baseballs).
Splitter grip should have the 'V' that is made by your index and middle fingers bisected by your thumb, and should be released with palm forward, toward the target, just like any other fastball. It is a fastball preset and release all the way.
Immediately after release of the ball, your forearm, wrist, and hand will go into pronation--on every type of pitch that you make.
It is really a shame that some people, who should (and probably do) know better, have tried to make a cottage industry out of "selling" pronation. Pronation is only mysterious because it happens so quickly in the pitching delivery that you cannot easily perceive it with the unaided eye and most pitchers are unaware that they do it, unless they look at quality slo-mo video of themselves.