For the perfect snatch to take place, every element has to be, in itself, perfect. Both technique wise, how the movement itself is performed positionally, and at which speed the lift is executed.
When talking about speed under the bar, it refers to the 3rd pull of the lift. The 1st and 2nd pull, the lifter is pulling to create upwards momentum of the bar, but for the 3rd pull, the lifter should be actively pulling him/herself underneath the bar. Following the triple extension, when the bar has achieved enough acceleration to continue it's upwards momentum without any extrinsic factor acting upon it, that’s the window of opportunity for the lifter to get into the receiving position, before gravity has won the battle against the bar, and starts making it move against the floor again. It’s the moment when the bar is “weightless” mid-air that you want to sweep yourself underneath.
Due to the fact, that gravity only acts at one constant velocity, it’s important to make the transition of the bar moving upwards and the body downwards ACTIVE! Shrug the shoulder to your ears and lead with the elbows following the triple extension. Make this transition as fast as humanly possible and it will increase your chances of making the lift.
There should never be a moment in which the lifter isn’t either moving the bar upwards or the body downwards. There is ALWAYS something going on whilst snatching, and NEVER a moment of relaxation nor hesitation.
This being said, just know it’s not an easy task! To be able to master the 3rd pull the precision on the timing following the triple extension to pull yourself under has to be absolutely flawless.