Instructions to that Diode kit.
http://www.jpcycles.com/instructions/pdf/ZZ38200.pdf
A diode would be like a check valve in a pipe, allows flow in only one direction. Though a mechanical check valve is known to have a little back flow until fully seated shut.
So as I understand it, with a diode and a ground at the turn signal indicator bulb connection it will allow flow only to the turn signal selected and still light up the indicator bulb. Without that annoying problem I have.