Why can swaps be positive and negative?
As the currency markets involve a simultaneous buying or selling of one currency to another, the guiding interest rate difference for the currency pair you are trading determines the outcome. For overnight positions, you are either levied a positive swap (the swap rate is added to your trade) or a negative swap (the swap rate is subtracted from your trade).
In theory, when you buy a currency with higher interest rate and sell a currency with lower interest rate, you are charged a positive swap. Likewise, when you buy a currency with lower interest rate and sell a currency with higher interest rate, you are charged a negative swap.