Best way of thinking about it is NOT in terms of currency. this is just confusing, although it does provide a very superficial explanation. It also provides a solution of how dynamic economy will adjust in order to balance the BOP but this is not something you need to know in HSC economics.
The best way of thinking about it, is in terms of an analogy using your household income. I.e. consider the following example. I will define what each item in my example is equivalent to before I give it:
CA
G+S Exports = Income from a job
G+S Imports = Purchases of consumables
Income Inflows = dividends from savings in a bank
Income Outflows = Interest payable on credit card
K&FA
Financial Inflows = purchasing something on a credit card
Financial Outflows = repaying the principal on a credit card
Lets say in week one you earn $100 in income from a job. Lets say you then purchase $110 worth of goods. Obviously you do not have enough income to pay for these goods so you have to get the money from somewhere else. So, you pay for $100 with your income, and the other $10 with your credit card.
Notice how in context to the BOP equivalents above it balances to zero? (i.e. CA = -10, K&FA=+10)
In the next week, you earn $100 and now have to pay $1 in interest repayments (lets assume 10% interest on the $10 owing on the credit card). Because you dont want to increase your level of debt, you only spend $99 on goods and services.
Notice again how both accounts are equal (in fact both accounts are zero because nothing has changed on the K&FA). (i.e. CA = 0, K&FA=0)
In week three the household earns $100 and now wants to pay off the credit card. So they only spend $90 on goods, and repays $10 on the credit card.
Notice how both accounts, once again - sum to zero. (i.e. CA = +10, K&FA = -10)
This is exactly what happens on our balance of payments however it is of course much more complicated.... I think this example helps people to understand how it works a whole lot better. The currency thing is the theoretical reason, but few students correctly conceptualise what is an abstract concept.