One of the great technical mistakes that many people are prone to in doing the clean is letting the barbell crash on them. That is, they receive the barbell too low in the front squat position. When the barbell the drops down on the shoulders the lifter will typically loose his posture or at the least have difficulties in bouncing effectively from the bottom. Ideally you wan't to be so technically proficient that you'll only fail the clean if you're simply not strong enough; even though you technically didn't everything right.
By combining the power clean and the clean in this drill the focus should, then, be to receive the barbell in a high front squat position in the power clean. A high front squat position is characterized by the upper body having a vertical or, more likely, somewhat forward tilted angle. The key is that the position of your upper body and the feet should be in a way that will allow you to continue to do the front squat without having to adjust anything.
Also, when you go from the power clean to the clean try to catch the barbell in the same position as in the front squat instead of rushing to the bottom position. Even when the barbell gets heavy or you get tired from the accumulation of lifts you'll typically pull the barbell higher than you think. Go to work!