Premixes don't exist on the earlier workouts (earlier than the Intensity Series, I think), but on later workouts, there are pre-programmed workouts based on the workouts on the DVD. They can be shorter or longer than the workout 'as-is,' repeat certain sections, combine segments from more than one workout, etc.
For example, on the PLB/PUB DVD, there are premixes for upper body 'up-only'pyramid, upper body 'down-only'pyramid, lower body 'up'only' and 'down-only,' and a premix (called "Bonus") that is a full body workout.
The KPC/L&G workout has several premixes for each workout, plus several premixes combining elements from both workouts.