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 relative to how high the barbell was pulled. When the barbell drops down on the shoulders the lifter will typically loose his posture or at 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 despite doing everything technically correct.
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 will typically still pull the barbell higher than you think.
When the pull is added before the power clean and clean it will present a two-fold challenge dependent on the weight chosen. If it's a relatively light weight the clean pull primarily has the function of practicing the pull phase and then applying it in the next two lifts. It's not uncommon that people finish the pull phase to early and rush too much under the barbell. When you only have to do the pull, it's easier to take the time to finish. If you just did it right on the first lift it's then easier to simply copy for the next two, even if you also have to jump under.
If you are a technically savvy lifter and loads the barbell heavy the added pull phase will do all of the technical stuff you read above but also take a toll on you and make you more tired for the next to lifts. This is a good way of practicing the clean in a state where you are not fresh for the lift - and the big challenge, of course, is to tighten your technique out of pure necessity.
Go to work!
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