James frame by frame
In the fitness world, we constantly hear the instruction to “stay in our heels.”  Particularly this instruction is attached to teaching the squat, the deadlift, and other movements where power application is quintessential.  In Olympic lifting there is some of the same carryover.  Applying power from the leg drive through the heels is essential to application of vertical power to the bar.  Think of the legs as the engine for the snatch, clean, and jerk.  Without leg power, a lifter would have no ability to create barspeed.  However, the potential energy stored in the legs is best applied through the connection of the heels to the floor.  A lifter with their weight in the heels creates a flat-foot to apply power through which allows for maximal ground coverage by the foot.  When a lifter shifts the weight from the back of the foot to the toes they loose that flat-foot and ground coverage.  This decreases the platform from which they can apply pressure to the floor and ultimately ends any power output.
However there is a time and a place to load the toes, namely at the top of the second pull or the top of the jerk drive when the ankle is extended.  Loading the toes at the top of these explosive movements is an indication of maximal power output.  Loading the toes early however will kill power output and decrease bar speed and height.
World Weightlifting Championships 2010
Here are two examples of what happens when a lifter loads the toes early.  The first is of Natalie Mclain’s 140 pound made snatch.  You will notice in her lift that she doesn’t extend the first pull long enough and as a result loads her toes early.  The second video is of Jeff Germond’s jerk.  You will notice with him that he loads the toes early on the jerk dip which causes the bar to move out and away from his body.  Both of these lifters are very good with the barbell and exhibit near perfect technique on a regular basis.  However, these lifts are good examples of problems in many lifters technique.
Natalie Mclain Snatch Analysis
Jeff Germond Jerk Analysis

Also, this is a flipping awesome article AND Sarabeth’s CJ only makes the article better.