This video illustrates concepts from the article in the July 2012 issue of Clean Run, "Footwork for Agility: Front Crosses" by Karen Holik and Eric Bobkowski.
"If you push off out of your front cross flat-footed, your acceleration will be much slower. Your push-off should feel like you are pushing off a spring board or like a tennis player waiting for a serve. Your “plant” foot now becomes just a push-off foot, which means your foot does not have to twist in order for you to take off to your next step. If you plant your foot any other way, especially flat-footed, at some point in the push-off to the next step you will have to twist that foot (remember all your weight is on that foot). Now you are twisting your knee."