HI , you don't get under the bar you need to pull yourself under the bar.
Its quite easy because as the barbell gets heavier gravity takes over and you can't pull the bar to that comfortable height you are accustom too.
So for each 1lb you add the barbell will be slighly lower and in the end you can't pull yourself under it.
Allot of people actually jump upwards to get under the bar rather than pushing/sliding their feet to the side which in turns also means you need to pull the barbell even higher and take the weight on the way down. A prime example is Dmitry Klokov, look at his feet.
2 good examples -