I find that split grade works particularly well with a contrasty negative. If I was to print unfiltered as a straight exposure, the shadows would block up before the highlight tone was right, or if I print for the shadows then the highlights haven't got enough exposure to show. Burning in would be necessary to get the highlights right, and that's not always easy to do, or necessarily repeatable exactly if multiple prints are required.
With split grade, I print the soft exposure first, to get tone in the highlights. I find that soft exposure does nothing to the shadows, and then when I make the hard exposure to bring up the shadows, it has almost zero effect on the highlights already exposed. So the highlight exposure in effect masks off the shadow, and the shadow masks off the highlight, and greatly reduces the need for dodging and burning. It's somewhat analogous to contrast masks that were used for printing cibachrome as ways of locally controlling exposure in certain areas of the print. So that's why it's called self masking, as a separate contrast mask isn't required.
BTW, self masking wasn't my term, that was from a course I took from Les McLean on split grade printing. He used to be active here years ago, but I haven't seen him post in a long time.
He talks about the effect a bit in this short article. He expands on it in his book.
https://www.lesmcleanphotography.com/articles.php?page=full&article=21