I will explain the first 6 bets, remember he has not included any commission with these bets.

First he uses a price of $4 and lays $1

1st bet wins $1
2nd bet loss, in lay betting you only lose liabilty not stake.
At $4 your liability is $3 since you won $1 on first race you are at -$2
3rd bet loss -$5
4th bet win $2 so is at -$3
5th bet win $2 so is at -$1
6th bet lost and since we are laying $2 our liability is double so loss $6 puts us at -$7

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