After visiting The Exorcist house (pics posted on this site), I am finally content after all these years to hear the best explanation of HOW Karras plummeted from Regan's side window to tumble directly down the Hitchcock Steps. Friedkin finally explains it more simply in the clip above and I thank our Cap for publishing it here (see 1:29 above).
While we've had the discussion on here before in which many (not all) believed that the Hitchcock Steps are parallel just below Regan's window to form a straight line, I now understand that the direction of Karras' leap was never dead even with the steps but rather it was a sharply angled lunge. Perhaps the camera angle and the editing have always confused me, but now it's clear. Huge thanks to the Cap for clearing up a mystery I could never understand even after visiting the house and the steps.