Correct knee flex with the driver is totally opposite of the knee flex needed for the green-side sand shot.
With the driver, I want just a small amount of knee flex. This helps me stay level through the impact and allows me to stand up nice and tall on the finish. Too much knee flex also contributes to slicing, because the hips are not able to clear properly through impact when the knees are too bent.
Have you noticed that knee flex makes a big difference in your game?