You’ve probably heard golf instructors talk about using the ground to create power. This phenomenon — known as ground reaction forces — is crucial for golfers who want to produce more speed.

Ground reaction forces have not always been well-understood, but they’ve always been present. You can look at swings from the legends of golf back in the day and clearly see them using their legs throughout the swing. From Ben Hogan to Tiger Woods, the legs have been the driver of the golf swing.

However, while the legs might be crucial for ball-striking, longtime GOLF Top 100 Teacher Peter Kostis disagrees that power is created by the legs.

“This will be controversial as hell,” Kostis says. “It is my opinion that the proper use of the legs, hips [and] body rotation — but especially from the hips down — the proper use of the legs is not to create power. I repeat: it is not to create power.”

So if the legs aren’t creating power, then what are they doing? According to Kostis, they are creating a stable base that allows you to swing powerfully with your upper body.

“It is to create a solid foundation from which you can swing faster,” he says. “The vast majority of your swing speed comes from the waist up.”

Once you have a solid base in your lower body, you are able to whip the club through the hitting zone much faster. But it’s imperative that you have a strong core and upper body to do so.

“I don’t jump up off the ground,” Kostis says. “But it’s a reaction to my swing speed, it’s not to create swing speed. I want my lower body to create a foundation.”

You can check out his full explanation in the video above.