I'm not sure about lately but Yihi used to have a second circuit board almost the size of a regular chip set for ecigs just to handle the balanced charging and battery monitoring. SX350J era mods.
That's what the pin is connected to or at least similar integrated into the chip set.

I may have been the first to incorporate this type of pin but that was for dual parallel mod carrying the - pole of the battery to the SX350 v2.
I likely have a better picture somewhere but the hole between the battery holes is for such a pin.

Oh damn it, wrong end of the mod but I'm sure you get the idea.