Rightward surplus votes
Those who were hesitating between Labor and Meretz but took comfort in knowing that their surplus vote would strengthen one of the two might find themselves voting Labor but giving Lapid an extra MK
Surplus-vote agreements between parties are designed to decrease the number of lost votes when a party does not achieve a full number of Knesset seats. If the two parties making a deal achieve a surplus greater than other parties, one of those two parties wins an extra Knesset seat.
There was a time when these surplus votes made a huge difference, and the Knesset even passed the Bader-Ofer law giving the larger parties advantages in calculating the surplus. All sides assumed that the president would choose the leader of the largest party to try to form a government. In 1981 Menachem Begin's Likud beat Shimon Peres' 48-47 thanks to 400 surplus votes; Begin remained prime minister.
In recent years it seems the importance of these votes has diminished, though parties with similar ideologies tend to sign surplus-vote agreements. Therefore the Labor Party's decision on Monday to shake off its image as a Meretz sister party and sign a deal with Yesh Atid is a bad omen. Shelly Yacimovich is pulling to the right; currently toward Yair Lapid, later perhaps toward Benjamin Netanyahu.
For now the move is tactical; in the future it may be strategic. Those who were hesitating between Labor and Meretz but took comfort in knowing that their surplus vote would strengthen one of the two might find themselves voting Labor but giving Lapid an extra MK. They thus might find themselves contributing to a Netanyahu government. The upside: Yacimovich is telling these people to stop making political calculations and vote according to their beliefs.