I used them on forest thinning crew in the '80s. We were thining out the trees so the remaining trees would grow up into 2x4s faster.
Our bow bars had guards on the top and bottom runs where you'd cut with a normal bar. The guards were pieces of aluminum stock bent to clear the chain, and bolted to the bar. They extended past the chain teeth and unlike the pics I see these days, extended nearly the full length of the top and bottom runs. There was also what they called a "stinger" on the bottom of the cutting face (the end of the bar), which was a steel bar that stuck out. You'd walk up to a small tree, plant the stinger on the tree and cut the tree. Any kickback force was taken up by the stinger against the tree. It was so stable and safe that you could take one hand off the saw while cutting and use it to push the tree over.
This setup was safer than the normal bar shape due to the guards. I once tripped while walking backwards through slash and had a running saw land in my lap. The chaps weren't even scratched. The other advantage was not having to bend over, which is useful when you're cutting a thousand or so trees in a day. The disadvantage is that the bar is limited to trees below 10" or so. Not a problem for this use, we rarely cut anything at large as 6".
I've looked at a lot of pics of bow bars since then and most aren't even the same shape ours were- they're shorter and the cutting face on the end is taller. I've not seen any with guards as long as we used and only a few with "stingers".