To add what everyone else has said together, you instinctively aim and you get good at that with a lot of practice. You need to learn how a sling works in order to release at the right time. You can see this when you give a slinger a sling of a different length than they're used to. Almost certainly their accuracy will be terrible because they're muscle memory is no longer applicable, it has to be adjusted to the new length which has a different angle and so a different point of release. The really good master slingers are the ones that can make the adjustment the quickest and that comes through experience.