A pyrotechnic cutter ?
I saw that many years ago. It was an innovation on sailing ships for cutting in emergency the wire guys when the mast breaks down (to avoid that the broken mast wrecks the hull).
Basically, it's a tube with a small hook at one end, to catch the wire rope. At the other end, inside the tube, a charge sends a hardened knife toward the hook, so quickly that the knife cuts throw the guy wire.
It's a one shot gear for this use, but it can be used in awkward position, with only one hand, with no fulcrum at all. Perfect for tree work.
Probably expensive for a job with many cuts, but I guess there is some industrial application with multi-use tools (like the nails gun for concrete or steel beams).