You're in a shoe shop, second in the queue for the till. Behind the shop assistant on the till is a pair of shoes which you have seen and which you must have.
The female shopper in front of you has seen them also and is eyeing them with desire. Both of you have forgotten your purses.
It would be rude to push in front of the first woman if you had no money to pay for the shoes.
The shop assistant remains at the till waiting.
Your friend is trying on another pair of shoes at the back of the shop and sees your dilemma.
She prepares to throw her purse to you.
If she does so, you can catch the purse, then walk round the other shopper and buy the shoes.
At a pinch she could throw the purse ahead of the other shopper and "whilst it is in flight" you could nip around the other shopper, catch the purse and buy the shoes.
But, you must always remember that until the purse has "actually been thrown", it would be plain wrong for you to be in front of the other shopper and you would be offside.
If that's too complicated to explain, try this diagram.