What it does
Minus raises the holder's Special Attack by 1.5× when a partner on your side of the field has Plus or Minus. In doubles, where two of your Pokémon are on the field simultaneously, the combo works exactly as intended.
In Champions singles, only one of your Pokémon is ever on the field at a time. No partner means Minus never activates — it's a dead ability slot, not an edge case.
Who runs it
Manectric carries Minus in about 3% of Champions sets. Almost every other build goes with Lightning Rod (redirects Electric moves in doubles and grants immunity) or Intimidate (cuts opposing Attack on entry). Minus appears mainly as a filler slot.
How to use it
In singles, there's nothing to do with it. The ability contributes zero in this format.
If you're playing Manectric in a doubles side event alongside a Plus partner — Plusle or Minun are the obvious ones — both activate simultaneously when they're out together, each getting 1.5× Special Attack. The combo is cute but rarely efficient at high-level play, since both slots have to commit to a narrow payoff.
Tips & strategy
Manectric's actual strengths are its 105 Speed, 105 Special Attack, and the field-control value of Lightning Rod in doubles. Minus adds nothing in a singles context. Seeing it on a set is a signal that the configuration isn't optimized for Champions.