Premixes take selected chapters and mixes them up in a different way. You can access them off the main menu of the DVD, push "play", and the remix flows like a regular workout.
For instance, when you have a circuit workout like Low Impact Circuit, which alternates between cardio step segments and weight lifting segments, but you only have time and want to do the weight-lifting, there's a premix for that. Or, if you only want to do the cardio - there's a premix for that also.
Hope that helps.
Diane