Bosch is one brand of a shielded spark plug boot. It is a metal boot that covers the plug. Electromagnetic waves generated by the spark are "captured" by the metal boot. The standard boot is rubber and radio waves travel through the rubber and can interfere with your receiver. If everything is installed properly, a shielded ignition will protect your receiver better than an unshielded ignition. It's standard technology on full-scale airplanes, 3W, ZDZ, and DA motors.