The overall idea here is that when the bar is racked before the movement begins, you should have your elbows slightly out ahead of the it. First, this helps to keep the bar on your shoulders and allows your body to support the weight (as opposed to strictly relying on your hands/arms to hold it). This is extremely important in lifts with a dip and drive component like the jerk or push press.
Second, this set-up allows for the bar to stay as close to your center of gravity as possible as it moves. With your elbows slightly in front of the bar, you are naturally going to press the bar up above you. If your elbows start behind the bar, your initial press will push the bar out away from you making it much harder to manage the lift. Getting this set-up into place before you take the bar off the rack is a good way to get comfortable with it.
Give it a shot the next time you’re working on any kind of a barbell press!