When you make a pledge on a project, your card will be charged 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 isn't successful then you will be refunded within 5 working days of the project closing unsuccessfully. An example of where this would happen is if the project is on the 'All or Nothing' funding option and doesn't hit its target, or the project owner decides to cancel their project.
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.