If you've noticed that a lot of Jaguar and Land Rover owners seem to be wearing Fitbits, it's not your imagination. Those little black rubberized bracelets probably aren't actually fitness trackers, however -- they're likely the British automaker's Activity Key wristband, and they've become much more popular than the company ever expected.

First introduced on the 2017 Jaguar F-Pace, the Activity Key is a simple, shockproof and waterproof wearable that allows drivers to leave their key fob safely locked in their vehicles and get on with their day. The idea behind the RFID-based tech, which incorporates a battery-free transponder, is that traditional key fobs can be inconvenient to carry around -- especially when engaging in the sort of lifestyle activities that SUV advertisements promise everyone is into. 

Whether you're kayaking, mountain biking, or shark jumping, you probably don't want to have to worry about losing or damaging your key (which typically costs hundreds of dollars). Activity Key is one way of solving that problem. Vehicles so-equipped can be unlocked by simply holding the wristband to a hidden sensor in the bodywork (e.g. the letter "J" on the Jaguar tailgate script), a process that automatically reenables the previously de-energized key fob locked inside, allowing the vehicle to be started.

Of course, Jaguar Land Rover isn't the only automaker innovating beyond traditional key fobs. Tesla's Model 3 comes with a similar credit-card-like RFID key, and even that's just a backup for the smartphone-based Bluetooth app, and other automakers, like Volvo, are taking similar strides with future car-sharing efforts in mind.

When the Activity Key option first launched, Jaguar Land Rover execs were hoping that perhaps 10 percent of buyers would shell out for the option. What they found is that on some vehicle trims, the purchase rate would end up much higher -- in some cases, upwards of 40 percent. 

Unsurprisingly, Activity Key is now offered on no fewer than six models for 2018, and more are doubtlessly on the way. On the Jaguar side of the house, the F-Pace and brand-new compact E-Pace crossovers are available with the tech, as is the XF Sportbrake wagon. Land Rover intenders can order Activity Key on Discovery, Velar, Range Rover Sport and Range Rover models.

As JLR spokesperson Nathan Hoyt tells Roadshow:

"Take rates vary considerably by model and even by trim within a model. As you might expect, we see lower take rates for these types of options on entry priced trim levels. For example, on the Discovery, the overall take rate for Activity Key is about 15%, but on some of the higher trim levels — HSE Lux gas and diesel models specifically — we see that increase to almost 30 or, in some cases, more than 40%." 

Especially considering that these wearables are priced at a healthy $410 to $510 (for most models) while leveraging simple, low-cost technology, Activity Key seems like a home run for the automaker.

To that end, it's almost surprising that JLR hasn't already expanded the range of Activity Keys to include different colors, materials and finishes, or to pursue co-branding opportunities with lifestyle and fashion firms (Tiffany Blue Activity Key, anyone?). Further, there's no reason that the tech couldn't be integrated into a line of timepieces, like the co-branded chronometers Jaguar has with British watchmaker Bremont.

Additionally, there's no technological reason that Activity Key can't also be engineered to allow for a car to be started without the presence of a key at all, or perform other vehicle functions. Like most automakers, JLR won't comment on future product plans, but Hoyt admits, "We are evaluating ways we can integrate this technology elsewhere and expand functionality or offerings."

In other words, whether you're a surfing junkie or just want to wear your favorite pair of super-skinny jeans, keep an eye on this technology.