I just don't see how you make any money - as you always end up even?
If you can work the maths through to this, you're one concept away from the "eureka moment".
In any betting transaction, where you back in one place (the bookmaker) and lay in another (the exchange), you will come out roughly even, assuming the odds are about the same.
So your first bet ("qualifying bet") follows this format - back in one place, lay in another, come out about even.
This first bet is called a "qualifying bet" because it qualifies your for a free bet.
So your second bet is made in the same way - back in one place, lay in another, come out about even.
Except the second back bet is made with the free bet you qualified for with the first bet. It doesn't cost you any money to make this back bet.
Possible outcomes:
- You win the back bet, having staked "free i.e. virtual money"?
You are therefore "up" at the bookies and "down" at the lay exchange.
- You lose the back bet, having staked "free i.e. virtual money"?
You have lost nothing at the bookies (you were betting with virtual money) and are "up" at the lay exchange.
You have lost nothing at the bookies
It is this first half of the second outcome that is key. Of the four possible outcomes, you have two "ups", one "down" and one "nothing happens". One of the "ups" is cancelled by a "down". The second "up" is not cancelled out.
You're "up" overall.