^The gameshow format? They have two "golden" container balls in front of them, one is the "split" and the other is the "steal", if they both choose split then they split the money, if one chooses steal and the other chooses split then the stealer takes all of the cash, if they both choose steal then they both get nothing.
It's a ****ty pointless version of The Prisoners Dilemma.
Dench I'm not sure if you're being sarcastic with "we know what would have happened if he was a fulllhead" but the reason the ball was in her court was because he screwed her over earlier in the game, he was desperately trying to win back her trust, and she didn't trust that he would split. So she thought she may as well steal and get everything, along with some revenge, and there was a good chance he'd try and steal as well so at least if he did, he wouldn't get anything.
Though I do think it's funny how Jasper Carrott says at the end with relation to the bald guy "a horribly familiar feeling for the other". Yes, he is bald, and used to failure and losing money to tricking women.
EDIT: Someone else explained.