Yeah, almost a cloaking device. Not quite invisible but more... unnoticeable, if that makes sense?
[He smiles at Chekov's attempt at levity, appreciating that his head is still very full of other, much more serious, stuff.
He looks down at the strap and smiles.] It's very important to have something like that. For a long time, I never had a home, never put down roots. This was the thing that always came with me. What I turned to to remember things. It still holds a lot of records. Some of them got corrupted at the same time as the vortex manipulator shorted out, but I've still got some things in there.
[He presses a few buttons and the tiny screen sparks into action with information about some random alien species. He shrugs.] This kind of thing came in useful when I was working for Torchwood.