I'll let Odyssey do the explaining, How-to video below:
We now have a dialed 3/8′s conversion kit available for our cassette hubs, which comes with everything you see above.This axle conversion kit will allow you to modify our standard V3 Cassette hub into an inbound 3/8′s axle with G-Sport Bolts. Rather than just sawing the ends off of our normal 14mm axle and drilling and tapping the ends, we re-designed the whole thing to use threadless slide on collars so that we could maximize the thickness and strength of the axle. The end result is an axle that is just as strong as the normal 14mm axle but has the clean flush mounting of a female design. The bolts are 3/8″-24 TPI with a 17mm hex head and a 6mm allen key broach just like you get on a Marmoset or Vandero 2 hub. If you don’t have 3/8″ dropouts, we’ve included spacer ring adapters for 14mm frames. This design will handle pegs just as well as the normal 14mm cassette axle.
You can find these at better bike shops, like CrucialBMX
Click below to check out a “how-to” on installing one of these bad boys that the dudes at Full Factory put together.