[vel0city]

The trailer shows the joycons sliding on that side with an additional attachment (see: 1:10). It seemed pretty obvious they were trying to hint at some kind of mouse-like optical tracking on a flat surface.

https://youtu.be/itpcsQQvgAQ?t=70

[delecti]

That could well be what those are, but to me they just looked like the wrist-straps, like the original Switch has. https://www.nintendo.com/us/store/products/joy-con-strap-gra...

[klausa]

They're both.