Thom I've looked at that spot for years wondering the same thing. I did as much research as I could on it many years ago and found a few things out. Perhaps there's more info on it now on the Internet, but 15 or so years ago here's all I could find out, and I'm not sure where I found this info. Anyhow that route seems to be called "Devil's Slide" by some. Not to be confused with Devil's Slide in Ocotillo Wells SVRA, or the lesser-used name of Devil's Slide for the major obstacle on Dishpan Springs trail near Lake Arrowhead.
So once you reach the bottom of Horsethief Flats there's a hill or knoll at the northeast corner of the little triangular-shaped valley, with a well-marked trail leading up to its summit. Sometimes our BBBB trail leaders take the group up to the top of that knoll but not always. From up there, there's a more faint track that goes north across a saddle to another ridge, where it follows a razorback and then the route plunges down a steep, loose embankment down to Arrastre Creek. This steep downhill is the section that is called Devil's Slide. From my research back in the day, and the only accounts I could find from people who traversed it, it is a VERY steep drop with very loose material so you pretty much slide the whole way. Then there's a sharp left turn at the bottom that can be treacherous. It is apparently a 1-way route because the slide is thought to be un-climbable. From the bottom, you cross over Arrastre Creek and it's an easy drive over to Bessemer Mine Rd and out to Hwy 247.
I just did a quick plot of the route from the top of the knoll down to Arrastre Creek and it's a half mile long with an elevation drop of 700 feet. The grade averages 28% across that distance, but the "Devil's Slide" section is about 37% as it drops 102' over 279' distance. The reason I've always chickened out over even trying it is that it's pretty dang steep just getting to where you make that final drop, across a narrow razorback. There's no turning around. You can't turn around once you've made it to the beginning of that final drop and assess it with your own eyes. You're pretty much committed as soon as you leave the top of the knoll.
If I recall correctly, you and I talked about this a long time ago when you were planning your Big Bear to Ballarat route.