Richard is right. The black conductor seems to have been added as an afterthought.
Wrapped around once on the existing splice and then the same yellow cap was probably reused.
The question with stranded wire...should you pre-twist? I know I do with solid wire because otherwise the wires will slip right out.
However with stranded wire I get everything lined up evenly and let the cap do the twisting for me.
Of course I usually deal with fairly thin 18 & 16-gauge wires inside small appliances (old blenders, radios, small fans), not 14-gauge stranded wire or bigger like what is shown in there, so I don't know if "size matters" also.
