Don't Move The Plates
It’s incredibly important to keep the bar over your mid-foot during the barbell lifts, especially as the weight gets heavier. Why? Optimal leverage diminishes as the bar deviates from this position. In other words, the lift becomes harder to execute, and it can be more dangerous on your body as it drifts from your mid-foot.
At times it’s easy to see the bar drift during a lift; other times, it’s a little trickier. Although this isn’t a sure-proof way to pinpoint movement, it’s definitely a good indicator: watch the words and numbers on the plates. (Yes, the words and numbers — the 45 or whatever brand you use.) Do they move from their starting position? If not, odds are you were in a good position and moved in a relatively straight line. If they drastically changed, you likely weren’t set or the bar drifted as you executed the lift.
A number of things can cause movement — poor setup, rocking the body forward or backward, hand displacement, inappropriate lean, etc. For the sake of a strong, good, safe lift, try to get into appropriate position while you work up-to-down and down-to-up.