Do you mean the baby swap or are you referring to the buses that take you to and from the parks. If you're talking about the baby swap, it's a little bit different on each ride and the CM waiting outside the ride can answer any questions you might have about it. The basic gist of it is that if there are two adults, a child too small to ride and/or another child, one adult can go on with or without an additional child big enough to ride and when that person comes off the adult who stayed with the baby can go on the ride with or without the child large enough by using the fastpass line (the CM will give you what you need to do so).
If you're talking about riding Disney transportation to and from the resorts to the parks, that's a completely different thing.