Welcome!
"Premixes" are when Cathe takes snippets of footage from one workout and reshuffles these snippets into a new mini- or maxi-workout.
For example, "Boot Camp" is a workout in which you have 8 complete cycles each of which has 1 minute of cardio, 1 minute of lower body resistance work, 1 minute of upper body resistance work, and 1 minute of core work. You complete all 8 cycles as originally produced.
The premixes Cathe put on the Boot Camp DVD include a cardio-only premix (in which she just puts all 8 cardio snippets together in one 12-minute fiendish string), a cardio-lower-body-only premix, an upper-body only premix, etc. That way you can do the same moves with different impacts on the body because you're doing them in a different order.
There are premixes in all of Cathe's productons on DVD only from Cross-Train Express ("CTX", produced year 2000) and later. And the premixes are one of the main reasons I adore her products.
A-Jock