How to fire and forget a subprocess?

I have a long running process and I need it to launch another process (that will run for a good while too). I need to only start it, and then completely forget about it.

I managed to do what I needed by scooping some code from the Programming Ruby book, but I'd like to find the best/right way, and understand what is going on. Here's what I got initially:

exec("whatever --take-very-long") if fork.nil?
Process.detach($$)

So, is this the way, or how else should I do it?

After checking the answers below I ended up with this code, which seems to make more sense:

(pid = fork) ? Process.detach(pid) : exec("foo")

I'd appreciate some explanation on how fork works. [got that already]

Was detaching $$ right? I don't know why this works, and I'd really love to have a better grasp of the situation.