When you unlock the $400 bet table, you also unlock the option to challenge your opponent to double the table’s bet. 
For example - if your current bet is $400 coins, challenging the opponent to double will give the opponent two choices:
1. Increase the current bet to $800 coins
2. Forfeit the game.
The first double is available for both players. The option to Redouble is in the hands of the player who accepts a double from their opponent. The maximum bet in a game can go as high as 64 times the original bet, depending on the table.

• Accept a double - If you accept a double request, your current bet will be doubled.

• Decline a double - If you decline a double request, you will forfeit the game and lose your current bet.

Redouble - Accept the double and offer another double in the same turn.
Find the maximum amount of doubles available for each table with the cube icon that’s on the table triangles in the lobby or the ‘Table Rules' button.