Ignoramus25480 fired this volley in
:

I thought that "tool balancers" have been around forever, no?

Not only that, but tool balancers on swinging trolly arms have been
around forever, too. I'm not sure I see the advantage of a big, bulky
spring arm attached to my tool.

That bit about the swinging trolley is important if you're moving the
tool around in a work envelope that might pull the balancer cable off-
angle from the reel. When it draws at an angle, it both fails to balance
the weight, and also tends to pull the tool toward the reel horizontally.

The arm swings an arc that covers the (say) NE-SW corners of the work
envelope, and the trolley allows it to move along the NW-SE lines.

I think that could be hammed up by an HSM faster and more easily than the
pantograph style arm depicted.

LLoyd