When you make a pledge on a project, the money will leave your account immediately. Your money will be held in escrow until the project closes successfully. Only once the project closes successfully will the money raised (including your pledge) be transferred to the project owner.
What happens if the project isn't successful?
Your money will not be transferred to the project owner until the project closes successfully. If the project doesn't close successfully then you will be refunded within 5 working days of the project closing.
An example of where this would happen is if the project is on the 'All or nothing' funding option and doesn't hit it's target. Or, the project owner cancels the project for one reason or another.
Have you changed when the money leaves a supporters account?
If you pledged on a project in 2017 or before, then you will have most probably been charged when the project closes. However, this has now changed so that payment is taken straight away when you make the pledge.
So no need to worry about making sure that there is enough money in your account when the project closes! However, if the project isn't successful you will be refunded as stated above.