The MPV was a RWD, not FWD. When doing 60mph+ on the local interstate and hitting some black ice, the right wheel would spin and the MPV would start to fishtail. Lifting up on the throttle would just cause it to slide in the other direction. Shifting to neutral allowed both rear wheels to get traction and cured the problem. Using the brakes was not an option.
Learning how to drive in a VW bug, back in the old days, I prefer oversteer to understeer. I know how to handle power drifting with a touch of oversteer, but have never learned or practiced power understeer. There aren't any good places around here to practice that. Besides, my 850 and V70 both have traction control up to 25mph and ABS. The MPV had nothing.
In the case of traveling on I-380 in the Penn foothills, probably doing around 70mph in a line of cars, down shifting to 4th and slaming on the brakes could have caused the rear to lock up momentarily and initiated a slide. I don't know, but it was obviously slippery somewhere to cause the lead car to go upside down. The downshift to 4th would not cause effective engine braking at that speed, which the poster will learn after a few hours of trial and error.
He did the only thing he could do, considering the traffic, and that was to go to the median to avoid the other cars. It is always amazing to hear of adrenaline based thought, where each second of real time seems to last 10 seconds.

Klaus
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1975 164 w/174,800mi (Sold) 1995 850T w/91,000mi, 1998 V70R w/129,000mi