Go on then, try to put your $1000 bet on Betfair and you'll get a rude suprise when they ask that you have at least $2600 in your account.
Well unlike the TAB, every bet has two equal parts, a back and a lay. To back something is obviously what you do at the TAB, you want something to win. To lay is to essentially play the bookie you will be liable to payout the win if the result gets up.
So what happens in this dropkick's situation is this:
He says he wants to lay Melbourne Storm for $1000 for $3.60.
Someone else (or multiple people) say they want some of that action, and take that bet.
Now if Melbourne Storm don't win, he wins that person's $1000 that they staked.
But if they do, he must pay out the $3600 ($2600 of his cash plus the original $1000 stake) that they are due, just like if they did at the TAB.
That is why I am saying he must have money in his account (his liability) so he can pay out if he loses.
http://help.betfair.com/contents/itemId/i65767722/index.en.html