Meaning you will not get something when you want it or someone doesn't want something for you. say you are selling an item and some one doesn't want it. they might say 'I'll buy it when pigs fly'. it just means you will never get someone to say yes to you when you ask for something.
(USA) Where the rubber meets the road is the most important point for something, the moment of truth. An athlete can train all day, but the race is where the rubber meets the road and they'll know how good they really are.
People whose behaviour is strictly controlled go over the top when the authority is not around, which is why most teenagers have parties when their parents have gone on holiday. The parents are the scary authority figures, but the cat's away and the kids are the mice partying and enjoying their freedom.