It uses an old heat gun (with heating elements removed) on a rotary dimmer to blow air into the bottom of the brake rotor. This rotor has a large offset, and is almost a brake drum in size. I fill that with coal or charcoal, and then put the top cover on. The slot lets me pass the piecework through to the flame.
I live in a really old farmhouse (1750) so it's very useful to have a forge so I can repair or duplicate any old hardware that I need. It's also really handy to be able to heat up large chunks of metal. It would have been really hard to get that cross bar hot enough with a torch to straighten.