Explained by Jason Day: Why you should increase your hip speed through impact to get you more distance
If you look at a lot of the world's longest drivers these days, myself included, we all rotate our hips very fast in the downswing.
Some people are born with natural speed but you can always work to improve whatever you have. I've done a lot of work in the gym to make my legs stronger so I can rotate faster through the hitting zone while maintaining complete control and balance.
There's no point doing something fast if you can't control it. I want no sway or lateral movement outside of my knee line on the backswing. I make sure my left hip kind of bumps towards the target a little bit as I start down towards impact.
Then as my arms move past parallel I can rotate powerfully from there with my right foot, right knee and right hip all starting to push and transfer the energy into the ball. I like to feel very solid at impact.
My hips have almost fully cleared and my weight is into my front foot. My arms and club shaft should be in a straight line, which helps deliver more power and hit the ball higher.