a) how powered and for how long?
Might even be passive, a tuned resonator. Heck it could be a bug, IIRC ironically passive bugs are (age-old) soviet tech. Imagine sweeping the whole frequency spectrum for resonators with 50s tech they were a nightmare to screen for.
and b) why a chocolate bar if so?
Maybe the idea is to track people for some time to gather intelligence on trench operations? Most probably aren't going to eat the thing right away.
Negative tax is a way to implement UBI. It's mathematically equivalent to paying out a flat sum and taking a flat tax which is easier to administer but then there's the political opinions of the mathematically uninclined asking "why are we giving billionaires money". (And yes both schemes are progressive, flat tax alone wouldn't be).
They're even in favour of doing it on the EU level, and argue that the TFEU already contains the language necessary for the EU to do it.