India, June 9 -- Through her revolt within the Trinamool Congress, Kakoli Ghosh Dastidar keeps coming back to a number - she says "nearly 20 MPs" are with her, against party founder Mamata Banerjee. The figure that really matters, though, is 19. It decides whether the rebels can switch sides and still keep their Lok Sabha seats, or lose them.

And it echoes a move made just months ago by AAP's Raghav Chadha.

Here is how it works.

The anti-defection law, set out in the Tenth Schedule of the Constitution, has the aim to stop elected members from jumping parties for power or money, or mainly switch loyalties when voters have elected them on a certain party's symbol. The basic rule is that if you are elected on a party's symbol and then lea...