Automatic payment means that when you hit the credit limit or at the end of the month they will automatically request the payment from your credit card. On the other hand manual payment means that you need to fund the account yourself, manually.
With manual payments, you pay in advance of ad activity. You make a payment first, then run your ads. Google deduct advertising costs from your balance as you accrue them. in Automatic payment Google will automatically charge you 30 days after your last automatic payment or when your balance reaches your billing threshold whichever comes first.
the best solution is to use VCC Virtual Credit Cards to prevent adwords from taking money from your credit card if you want I can help you with this.
Through automatic payment your card will be automatically charged for the amount payable to adwords while in manual you will have to enter card details and send the payment