Pick the closest bone to each spline ring. For example, the base of the forearm (first spline ring from elbow) would be assign to the "right/left_forearm_bow_0_geom" and the last spline ring (closest to wrist) would be assigned to the right/left_forearm_bow_3_geom. And any spline inbetween would be assigned to the "right/left_forearm_bow_1&2_geom" and weighted to fine tune the twisting. The bicep, thigh and calf are setup the same way. These bone are for the bowing feature, also the twisting as mentioned.
Green monkey has this installed and David has the squetchy Sam model for reference.
Also, once you run the install rig plug-in, only geometry bones show in the modeling window.
I usually hide the shoulder fans, stomach IO and the chest IO bones, I use them just for weighting purposes.