Okay, I joined Betfair, but havent put any money in yet, i need to know how it works first, and i cant find any information i understand

In the next at Sandown, Gone critical i see that you can back it at $6

But then you can lay it at $6

Therefore am i right in assuming that i could have $100 on gone critical, and then lay it at $6 for a hundred also, and whether it wins or loses i collect $600 back??

Something aint right, can anyone help, i know its probably the stupidest question ever..........

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good luck and happy punting