In New Zealand, we have a Mixed Member Proportional system.
Voters have two votes. The electorate vote is used to vote for a candidate to represent their electorate. This vote is determined on a first past the post basis: the candidate with the most electorate votes, for that electorate, gets the electorate seat.
The party vote is a vote for a party and is used to determine the proportional make up of the total seats in parliament. E.g., if a party gets 30% of the party vote, then they should hold 30% of the seats. If less seats were earned through the electorates than the number of seats earned by the party vote, then the difference is made up of list seats.
Example: There are 100 seats in parliament. There are 90 electorates. The Red Party wins 10 electorates, and earns 12% of the party vote. They will be represented in parliament by 10 electorate MPs and 2 list MPs.
There's an additional complication of overhang seats. Overhang seats occur when a party earns more seats through electorates than they do through party votes. In this case, there will be extra seats in parliament.
So for example, if Red Party earns 10 electorate seats but only 5% of the party vote, they will be represented in parliament by 10 electorate MPs and parliament will have 105 members.
The question is – what prevents a party from gaming this overhang dynamic by splitting the party into two, one party to attract party votes and one party to gain electorate votes, and instructing their supporters to vote accordingly?
For example, if the Red Party splits to the Dark Red Party and Light Red Party. The Dark Red Party wins 10 electorate seats and 0% of the party vote, and so is represented by 10 electorate MPs. The Light Red Party wins 0 electorate seats and 10% of the party vote, and so is represented by 10 list MPs. This effectively doubles the party's representation in parliament.