If you want to have good leg drive out of the bottom of a squat, it starts with having a neutral spine, and then driving down into the floor, rather than trying to stand up or push into the bar. When you try and push into the bar, you will extend the lumbar spine and lose the glutes. Regardless if the goal is quads or glutes as shown here, you have to have a neutral spine at the bottom to generate a lot of tension in the respective muscles.