That's a scary story that you related, Bird, no doubt about it. And it can happen to anybody who's not being careful. One time you do something like that, you "accidentally" live to tell about it, and another time, you don't. Need I mention that with an HST transmission, if you for any reason stop pushing the pedal (like if you fall off or scare yourself witless when the tractor rears up) the tractor will just slowly lower itself to all four wheels again?
I routinely get to the point when pulling trees that all four tires just spin, even though I've easily got more than double the traction my tractor had stock. So if anyone runs the risk of flipping the tractor from traction, it's me. But, as Wen mentioned, there's no instability at all when I'm pulling from the drawbar. However, one caveat that probably shouldn't even need to be said, but it's best not to take anything for granted in the area of safety: Make sure the chain, cable, strap, rope, whatever, extends directly from the drawbar to the object you're pulling. I had a helper pulling chain for me one day and it was toward the end of the day and we were all tired, so he accidentally ran the chain over the top of the rear blade instead of under it and I didn't notice it. So when I pulled forward and the chain tightened, the tractor almost instantly reared up on me. It was time to knock off for the day anyway... /w3tcompact/icons/wink.gif
And, again, I'll second Bird's comments about the effect of something you're pulling suddenly releasing. You might get a little jolt when the tractor lurches forward, but that's all. What you have to worry about is the object you're pulling suddenly getting hung up on something. Then you could be in for a nasty ride.
Mark