As a number of people have already said, it would be the second pull. The reason being that at this stage the lifter has rebent his knees (which are reasonably straight from the first pull of the floor)and is essentially in the jumping position. From this position, the lifter simple jumps, shrugs, pulls, catches and absorbs. Think about perfroming your cleans from a hang position (high above the knees to begin with, and below as you progress), as this incorporates a stretch shortening cycle as a vertical jump would.
I would suggest that you break the clean up into stages and learn each before progressing to the next. A simple progression would be (top down from a high hang): shrug, high pull, catch (full hang clean). Spead a while at each stage, always ensuring great technique, and trying to move as quickly as possible.
You may also what to learn the snatch. This can be learnt in the same progressive manner as the clean, and has the advantage that the catch possition is easier to perfect. On this subject, when racking/catching the bar in the clean, ensure that you catch it on the shoulders with a loose grip, and not simple with the wrists in front of the body. A simple way to ensure you are doing this correctly, is to perform a front squat once you have caught the bar. If the bar is up on the shoulders, this will be easy, if it is out in front of the body you will know about it.
This is a common error, which can be preveted providing the movement is learnt properly in the early stages. To get the best out of these movements, I really encourage you to spend the time practicing the techniques before perfroming the whole lift and loading it.
I apologize if i have strayed from your original question a bit. I hope the above information will be usefull.