The NBA players union was scheduled to meet with a mysterious wearables company today to decide whether to allow the use of fitness trackers on the basketball court. But what exactly is Whoop?

Matthew Dellavedova was told at the end of March that he could not wear his Whoop fitness tracker while playing for the Cleveland Cavaliers, which ESPN said has sparked debate behind closed doors about whether it’s time to open up the NBA to hi-tech gadgets.

The Whoop Smart Trainer (apparently pronounced like “hoop”) is almost unknown to the general public. It’s like Fitbit, but for super-elite athletes. It tracks a range of vitals to offer unprecedented insight into how hard the wearer is pushing for the burn. All that data is fed to the coach’s computer, so that they can push them even harder.

It’s been out since September 2015. Normally, gadgets get covered by the press, poring over every detail and noting down every nook and cranny. But nerd-level reviews of the Whoop are notably absent from YouTube.

In fact, there are almost no public videos of the Whoop online, except a carefully-choreographed explanation hosted by the CEO.

The wearable hooks up to an app that, at the start of every day, gives a recovery percentage. This indicates how ready the device thinks your body is to get training today. Throughout the day, Whoop is keeping track of a variety of variables, like heart rate, body movement, and skin temperature.

It’s in a totally different field than Fitbit. Fitbit has a whole range of trackers to appeal to individuals, and the company does sell trackers designed for serious athletes. The $249 FitBit Surge, for example, comes with GPS for tracking runs in detail.

While all Fitbits track steps and movements, with a website for tracking how a group of wearers are doing, most of them aren’t really designed for intense athletics, where the sole focus every day is training. Nor does it provide the same coaching tools as the Whoop.

Dellavedova, sans a Whoop wristband.

At the end of the day, Whoop can give the coach a breakdown of how hard the athlete has been working throughout that day, while telling the wearer how much they should sleep to recover. The “strain” score is used to work out the recovery percentage for the following day. An easy day yesterday might translate into Whoop giving a high recovery score, pushing the wearer to work harder.

And that’s it, really. Nobody’s sure when, if ever, the Whoop will be available to the general public. Some people still aren’t totally sure what it is. There’s a form on the website to express interest, but no indicator of how much it costs.

With its focus on constantly bettering elite athletes with 24/7 tracking, it may always stay a mystery to the public.

Photos via Patrick Smith/Getty, Jason Miller / Getty