Controllers that let you reach out into the "mixed reality world" are a big deal for the future.

The final Build keynote surprise was a love letter to everyone excited about Windows Mixed Reality, and it came in the form of motion controllers that allow you to reach out and "touch" the virtual world. These controllers follow a lot of the same rules as the Mixed Reality headsets that are expected later this year, which means they have new tech that will make them unique in the world of VR motion controllers.

If you were excited about Mixed Reality before, this addition will have you scrambling for your wallet. Here's what you need to know.

Inside-out tracking

Just like the associated headset, Windows Mixed Reality controllers will not require independent sensors to interact with the virtual world. The controllers rely on the same inside-out tracking that's needed for HoloLens to function, which means you get the same six degrees of freedom (6DOF) motion control seen with Oculus Rift, HTC Vive and PlayStation VR, without any of the extra sensors. The only thing connected to your PC is the single cable coming from the headset, so there's none of that extra mess.

Acer Mixed Reality

This works, in large part, because of the sensor array on the headset. As long as the cameras and sensors can "see" the controllers, they will be positioned exactly where your hands are in the real world. As we've seen with HoloLens, that field of view isn't exactly as wide as what your eyes can see. But it's close enough to have a lot of fun.

Microsoft says the sensors on the headset will allow the controllers to be tracked slightly beyond the camera field of view, but right now it's not clear how far beyond. You may not be able to reach behind your back to grab a virtual shield or another arrow from your virtual quiver, as you can with an HTC Vive controller. But everything else you can do with an existing motion controller you can do with these new controllers. At a minimum, that means existing VR games and apps could be ported to Windows Mixed Reality if their developers are interested.

Different controllers for different folks?

Like the Mixed Reality headsets, Microsoft's partners are the ones doing the design, retail, and shipping of these new controllers. It's not clear if there will be one unified design for these controllers yet, or if, like with the headsets, there will be different designs based on what the OEMs think is best. But the inside tech will be the same.

Currently, the only manufacturer that has announced controllers is Acer. These new controllers are expected to be available in a bundle with the new Mixed Reality headsets later this year for $399, but developers are now able to preorder the bundle with an expected ship time this summer.