Throttle servo likely due to proximity to ignition module/wiring.
Similar situation on some ESC's SBEC.
The issue is conducted noise on the (long) leads causing internal interference in the servo or receiver.
By installing the ferrite core on the leads, the unwanted high frequency noise voltage is reduced (dissipated as heat at an inmeasureable level) and the interference is eliminated or improved.
You see ferrites everywhere ... mainly on computer cables, power supply charger cables, etc ... they are normally overmolded in plastic on the cables.
In RC, the only thing we need to do is secure the mass of the ferrite so it doesn't cause the wire to move around.
that is a good point, making it secure and not flying around. pun intended !! I guess I won't get involved with them as I have very short leads, not over about 15" long.
thanks for all the input, great help.
sticks