Wanted to show using videos what I was talking about a fair bit this Sunday, which was the path the bar travels during the pull from the floor to the point of explosion. The sole purpose of the first pull is to get the bar from the floor into the optimum position for driving it explosively upwards. If the bar is not in the right position, then despite the lifter's best efforts at explosion and then the pull under, the bar will not land in the right place, either resulting in a miss, or a struggle to stand.
The bar must move slightly backwards and into the lifter as it rises during the first pull. To allow this, it must start over the middle of the foot (around the laces on the shoes), not right up on the shin. The lats should be engaged slightly in the start position to stop the bar drifting out. As the bar is lifted, the weight should be on the heels. The knees should be pushed back and flared out to the sides to move the bar upwards, with a focus on keeping the back angle exactly the same. Once the bar reaches the explosion point (mid-thigh on snatch, a bit lower on clean), then the jump/hip thrust is initiated to drive the bar upwards.
Watch the video below and note the path of the bar. It comes in on the first pull, then goes out slightly due to the explosive contact of the hips. Its movement is always upward primarily. The pull of the arms drives the lifter under and brings the bar back and over the centre of gravity.
In the below video, the bar path is forward off the floor. The lifter is too far over the bar, with his weight well on his toes and the pull is rushed. As a result, he jumps forward and has to fight to catch the bar. At a heavier weight, this would undoubtedly result in a missed lift.