What is Humbuster
It's a cable design by Fractal Audio to reduce unwanted noise due to ground loops. Humbuster cables require Humbuster-compatible jacks on the Fractal Audio side of the hardware. It can be used with all 1/4" outputs on the Axe-Fx II, III, FX8 and AX8.
A Humbuster cable has a TRS end and a TS end. The TRS end ("stereo") goes into the Fractal Audio device. The TS end ("mono") connects to the side, like an amp.
"If you pull the cable out halfway it will get twice as loud. This is due to the humbuster output design. Don't pull the cable out halfway." source
"The Humbuster outputs sense the ground noise at the far end of the cable and cancel it out. Remember there is no such thing as a wire with zero impedance." source
"The 1/4" outputs have a "remote ground sense" feature. You take a TRS cable and on one end you put a TRS plug and on the other end you put a regular TS plug. On the TS end you tie the ring conductor to ground. Plug the TRS end into the Axe-Fx II. The ground noise of the attached equipment is sensed and added to the output signal thereby cancelling the common-mode noise (hum). This is especially useful when doing the 4CM since that's when you usually get the worst ground loops."
"Won't do anything for pickup hum. It just fights ground loops." source
"Humbuster cables do not eliminate noise. Humbuster cables in conjunction with Humbuster outputs can significantly reduce hum due to ground loops. The outputs of Fractal Audio products have a special circuit that, in conjunction with a Humbuster cable, senses the difference in ground potential between the devices and cancels the difference thereby reducing hum. It's certainly not snake oil and you can find numerous testimonials on our forum that speak for the effectiveness. source
Which products support Humbuster
The Axe-Fx III, Axe-Fx II, AX8 and FX8 have Humbuster-compatible outputs.
"The FX8 is designed to run Humbuster cables on all of the outputs. If you are running mono, you will only need to run two Humbusters: one to the front input on your amp from Output Pre on the FX8, and one to the FX loop return from Output Post on the FX8." source