Picture and measurement of back bevel????

[Dick Strauss]

Does anyone know what a back bevel is? Others have mentioned that a back bevel on the blades/knives does a better job of jointing and planing curly maple. Does anyone have an illustration and measurements (what bevel angles work best) that they might post here so I can better understand? I want to have them reshaped at my local sharpening shop but need to know what I should ask them to do.

FYI-I don't have access to anything with a Byrd carbide head.

Thanks in advance,
Dick