They need last 6 months statements of your bank account. If you can wait for that long from the time you open the account till you apply, and the last 6 months statement doesn't show any sudden increase in funds then no documentation or explanation is required.
The most common use case is, people have really old account, ideally from the days you've started earning, and they submit a gift deed from parents once the money is deposited.
The POF requirement is to ensure you (and your family) has enough to survive on basic means in Canada once you arrive even without a source of income for an economic immigration stream. If you manage to get a job, then this requirement is waived off, although this a very rare scene. So, if no job offer, then you have to prove that the prescribed min. amount is met, available to you at the time of application and you have access to it.
The gift deed proves that you don't have to return the gift money and it is yours, so it's ok to show it getting credited anytime in the 6 months statement.