Many people learn how to do the power clean from the hang (starting just below the knees). When you move to the power clean from the floor, there's another technique issue you need to watch out for. You need to initiate the pull from the floor with the hamstrings.
Move your knees back so when the bar reaches the knees, the tibias are perpendicular to the floor. The bar is kept close to the body while you're doing this, so it's lifted in a slight backward arch.
When the bar reaches the upper third portion of the quadriceps, explode up with the legs, hips and lower back. "Stand up" as explosively as possible.
Don't pull with your arms too early and don't accelerate until you pass the knees. A common mistake is starting too fast off the floor. Notice that the feet leave the floor, but it's not an upward jump. Instead, it is a lateral re-adjustment of stance width.