An automatic transition is just a regular state transition with one difference: whenever an item reaches the From state of the transition then it is executed automatically, without human interaction. This means that all the workflow actions are executed, all guards are checked just like with any other state transition.

To create an automatic state transition you just have to check the Automatic checkbox on the transition configuration dialog:

Automatic transitions are marked with dashed lines on the workflow diagram:

The only restriction for automatic transitions is that you cannot define more than one of them with the same From state.