Breaking the Bar from the Floor
The separation of the barbell from the floor in the snatch and clean should be relatively smooth—that is, it should not be jerked abruptly from its static position on the floor. This kind of separation creates two potential problems: first, it significantly increases the likelihood of an unwanted shift in balance or position; second, if the initial speed off the floor reaches a certain threshold, the lifter will have to actually slow the bar down somewhat to maintain tension against it and continue applying force effectively. The heavier the weight, the more the first issue is a concern, but the less the second issue is probable or possible because the lifter will simply be unable to achieve that level of speed.
This should not be misinterpreted to mean that the separation of the bar should be slow, but simply that there needs to be continuous tension and force application without the allowance of slack, and inadvertent shifts in balance and position need to be avoided.
As with nearly everything in weightlifting, there are exceptions to this general rule—some world class lifters are able to consistently execute successful lifts with a vicious yank of the bar from the floor. However, like most things, experimentation with this approach to the start should be reserved for after a solid base of technical proficiency has already been established and significant training experience accumulated.