I think the whole earth thing has added confusion to you.

Try think of it more as a simple circuit. One your bike the frame acts as part of the circuit on the return side to the battery (electron flows negative to positive) so whether it be the reg rec or the battery tender lead, the red always goes to red at the battery and the black always goes to the negative. Depending on whether you have an ammeter or not determines on whether your regrec can connect directly to the battery or the other side of the ammeter to the battery.

I’ve tried starting bikes up when they are on an opti-mate before now, usually the optimate just cuts out until you unplug and plug it back in again. It hasn’t damaged the optimate, the battery or the reg/rec. what I have had is where the battery has been fitted the wrong way around and this has blown the fuse when the revs have picked up. (But because of the fuse saving the day it hasn’t caused further problems)

Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)