You simply attach B to a zapper and it will zap him no matter how far he is from it. For example: Spoiler: Demo
That's not entirely true. The zapper will only zap B if it's near B's center of mass. Also the lightning always connects to B's eye. So if you make a very large B with his eye very far apart, and put the zapper near the center, you'll see the lightning effect.
I tried to understand @richardfu_'s explanation in a practical way: Indeed, seems to be the object's center of mass (CoM) rather than the geometrical center of the object. Next question would be how heavy different block types are. Ok, at least relative to each other. Surprisingly for me, there are two other things in the video aside from the lightnings to notice: Rubberband limbs happen also to single bots and it seems because of their spacial extension beyond the one block-sized area they normally stand on. Before I thought that it would be necessary to connect at least two bots for the effect, or do even worse things to them. No, one single bot is sufficient for rubberband limbs. In the Video B has something 16x16x16 sized as head, which is just nice to see the effect (according to its facing direction (East) the bot and all attached blocks undergo the initial rotation before the clock starts, but the positions of hands and feet don't). With smaller extensions the effect becomes smaller, and with an extension that exceeds the normal standing area only by one block the effect is quite difficult to notice at first sight. But with knowing all this it is possible to see it. The CoM of the extended bot seems to define the rotational axis for the initial rotation before the clock starts. But it is not yet clear to me in which exact way the CoM is defining this axis.
I know, this message is exceeding the original intention of this thread a little, but take it as an example application of the Zapper-Lightning-CoM interaction outlined above and as an attempt to answer my last bullet above (how CoM is defining the rotational axis). In the videotaped level the CoM is known because of symmetrical desing in two directions and because of the Zappers measuring it in the other direction. And it seems that the answer with regard to the rotational axis is the most simple and direct one. But things getting worse when introducing greek mythodolgy to this level. When the three Graeae bots are sharing their only eye, the Z gods no longer can see this single eye and will fling their lightnings at B Graia directly.
Deprecation notice: I tested my own "Center of Mass" card from the "Thunder Lab (CoM)" video above (QR in the video) and found that it does not work any longer as originally intended with the most recent app version (Android tablet). I also think that the "Thunderstorm" card above from @Frenzies worked back then differently than it does today. Also cannot remember that the three Graeae bots have been this furious as they are now. Improvement notice: The two card variations from my "Thunder Lab" video above even work better with the updated app since from the different behaviours in the video apparently only the thunderstorm effect remained - at least this is my first impression. Spoiler: Thunder Lab (spatial) Spoiler: Thunder Lab (mass) Spoiler: Center of Fury (some reconstruction) Spoiler: CoM Lab (spatial) Spoiler: Com Lab (mass)