Imagine a device that trains your brain to see the world around you in slow motion. It enhances your reflexes, and trains your brain to anticipate things before they happen.

That’s the idea behind the updated version of an old tool (that some NBA stars and executives are reluctant to talk about). But the device has recently gained publicity, thanks to a recent ESPN article.

They’re called strobe glasses, and they’re fascinating.

Stephen Curry, the reigning MVP of the NBA, used the glasses to help him raise his scoring average from just under 24 points per game in the 2014-15 season to over 30 points per game the following year.

He also shattered his own record for three-point shots, going from 286 made three-pointers to 402.

That’s an increase of over 40%.

In one year.

Over the previous record.

How is that possible?

Strobe-light Training

It all began decades ago with the man who many of us basketball nuts still regard as the greatest of all time: Michael Jordan.

The ESPN report explains:

In his Chicago Bulls heyday, at the peak of his powers, Michael Jordan declared that he wanted to train with strobe lights–like, from a nightclub. His longtime trainer, Tim Grover of Attack Athletics, says that to understand why, you only have to watch old video of Jordan.

“Go watch old clips of MJ at the free throw line,” Grover says over the phone. “Make sure it’s in-game. … OK, do you see it?”

See what?

“The flashes,” Grover says.

Once you see it, you can’t unsee it. It’s the worst secret ever. The blinding light of photographer’s flashes obliterates the screen repeatedly throughout some of the most televised moments in history. How can a man shoot with all those blinding pulses of light?

In an effort to help Jordan cope with all those undesirable camera flashes, Grover set up strobes in the gyms where Jordan would practice. (Players who joined the scrimmages were sworn to secrecy.) Eventually, the trainer found a device called “Strobe Spex” glasses that Jordan could wear, simulating the effect.

But then, a funny thing happened.

Jordan noticed other positives from using the glasses. “The game seemed to slow down,” reports ESPN. “[Jordan] picked up on visual cues he wasn’t seeing before. For reasons he couldn’t quite explain, it was making him better, even beyond tolerating photographer’s flashes.”

Years later, after Jordan retired, Nike produced its own pair of glasses known as the Nike SPARQ Vapor Strobe. They used internal LCD lights, set to flash at adjustable speeds, to distract the brain and force it to use whatever visual information it could gather to accomplish its tasks. (One expert compares it to a form of resistance training, except instead of working on the physical side, the glasses train the brain and its ability to process information.)

Nike even commissioned Duke University to conduct a study on the technology, which revealed the glasses boosted short-term memory and perceptual ability.

But some wondered about potential side effects the study didn’t consider. Virtual reality systems, which operate on a similar principle of showing the brain single-framed pictures, have been known to create “simulation sickness,” in which a user develops temporary dizziness or a loss of balance (among other potential dangers).

Interestingly, for an undisclosed reason, Nike shut down the SPARQ program in 2012.

But Steph Curry’s personal trainer, Brandon Payne, swears by the Eclipse goggles (made by Sensory Performance Technology) that Curry uses when training.

(You can see Curry and the goggles in action at the 2:10 mark in the following clip.)

“I hate it when people call it ball handling drills or working on hand-eye coordination,” Payne told ESPN. “That’s an oversimplification of it. This is neuromuscular efficiency…Remember, we’re looking to improve by hundredths of a second. The difference between getting a shot off and not getting a shot off is all about fractions of time.”

“We cannot be wasteful.”

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A version of this article originally appeared on Inc.com.