(I am not an expert, so don't flame me if this account contains
howlers). In general, 3rd rail, (non-Underground) emus have what is
called a 'bus line' linking all of the shoes (on both sides) (via fuses
and the 'line breakers') within each unit. Therefore the shoes on the
side of the unit away from the juice rail are likely to be live &
should be treated as such. In this way, as long as one shoe of a set
touches the juice rail, all the motors of that set receive juice. If
multiple shoes of a set are receiving current, that current will divide
itself up between the shoes more or less equally, in this simple
description. Should a shoe encounter a gap, its share of the total
current is not instantaneously transferred to the other shoes. There
will be an arc, and more current will start to flow through the
remaining shoe(s). Once the arc becomes extinguished the whole of the
current demand will be carried by the remaining shoe(s).

If a section of third rail has to be isolated to allow work to take
place, that section could be energised by an emu's shoes and bus line
bridging the gap. I think I read somewhere that staff used to become
aware of this happening if MG sets on nearby emus were heard to start
up. (Do modern emus have MG sets?) I expect there are monitoring
devices. Or shorting bars to bring out the breakers. Didn't Underground
workers have 'lamp boards' with sets of bulbs in series?.

The bus line is not carried between sets running in multiple however.

Units intended for tunnel use such as Class 313 units, and LUL stock
are not allowed bus lines. Reasons are mainly to do with fire risk I
think. I seem to recall there was a nasty fire in the Paris Métro in
the 1900s blamed on insulation breakdown & arcing on wooden bodied
stock.

I think that the shoes attached to bogies on LUL stock only feed the
motors on that bogie, but I stand to be corrected.