This sprocket (cogs mesh with other cogs) bolts to the ISO (six bolt) disc mounts on a mountain bike hub in place of the brake rotor.
With a 135mm wide frame/wheel, you can then ﬂip the wheel to get a ﬁxed gear; you may need washers to tweak the chainline. With a narrower frame you’ll need a re-spaced front disc hub – which LFB sells. Why not just use a normal ﬁxed rear hub with a lockring?
The main advantage – apart from the secure ﬁtting – is that in 135mm frames it allows unusual ﬂip-ﬂop combinations. You could have a ﬁxed cog option for your singlespeed mountain bike, for example. Chain tugs help.
The ISO Cog comes in 1/8in and 3/32in in 16-19T.