The Split-S is essentially an inverted Immelmann turn.
How to fly it: Starting with straight and level flight at a higher altitude, the airplane is rolled through 180 degrees at the start of the maneuver. Up elevator is applied as soon as the airplane is inverted, and the throttle reduced. The airplane then enters an ‘inverted’ dive and is flown towards the ground.
Keeping up elevator applied, the airplane is pulled out of the dive and returned to straight and level flight to exit the Split-S maneuver. No rolling out is necessary, as the airplane will already be the correct way up.
Above is the Split-S on video (on the RealFlight G4 rc flight simulator), including close-ups of the Tx stick inputs and the airplane’s response.