The DNS change request works like this:

1. You contact a domain registrar and request the change.

2. That change is sent to the registry (the backbone that stores all DNS info) for submission to the root servers (the network of servers that make up the world wide DNS database).

3. Each of the root servers add the change to themselves. This process could take up to 72 hours (in some extreme cases, it can even be longer).

4. Depending on the area in which your Internet connection is located, you will access one of the root servers, and if it HAS been updated, you will be pointed to the new DNS info. If it has NOT been updated yet, you will be pointed to the old DNS info.

So, depending on where you are in the world, the process could take between 24-72 hours. It is not because it requires human interaction, but rather you are waiting for all the root servers to update.