Tyler, I think you're referring to the re-bend of the knees as you go into the second pull (the jump and full extension) As you pull off the floor the bar should be pulled in close to, if not touching touching, the shins, the back angle should remain close to constant at this point. As the bar clears your knees, the back angle opens up, the knees dip slightly and the hips extend. Think of the starting movements when jumping from a standstill.
The bar trajectory should be a straight vertical line. In actuality it isn't completely straight, but you should attempt to keep it as straight as possible.