Front squatting is a highly effective method of strengthening the legs for the recovery from the clean and can also be used to condition them to catch the weight in the clean position.
The lifter begins by taking the bar from a rack and assuming the same position that he (or she) will be in the end of the clean. The lifter descends smoothly into the full squat position and then recovers immediately to the starting position. The torso is strictly vertical, the upper back straight and the lower spine slightly arched throughout the lift. The knees stay in line with the feet while sqautting (neither going inside nor outside the feet during the descent or acsent).The feet are always kept flat on the floor during this and all other versions of the squat. Squatting on toes places undue strain on the knee joints and gives the lifter only a very small balance point. The lifter should try to descend in a controlled but smooth manner and should avoid crashing (i.e., dropping in a completely free fall without any control on speed) or rebounding out of the full squat using the joints. (He or she should feel the muscles doinbg thee recovering instead.) However, it is generally a good idea for the lifter to attempt to explode up from the bottom of the squat with as much power as possible. This effort will build power in the squat but will not place as much stress on the joints as crashing into, or bouncing up from, the bottom position.
Variations of the squat include the squat with a slow descent, the squat with a pause at one or more positions in descent, the squat with a pause at the bottom and the squat done more quickly than usual (in the descent, the turnaround into the recovery and/or during the ascent).
source: The Weightlifting Encyclopedia: A Guide to World Class Performance by Arthur Drechsler (buy here)