what's short fixin?

The title says it all.

Leader leads a pitch, gets to the anchor and "fixes" the rope. Once this occurs the second jugs the fixed rope while the leader simultaneously starts leading the next pitch with the remaining rope. The leader either self belays, with a silent partner for example, or essentially solos until the second arrives and can put the leader on belay.

Basically allows for more or less constant upward progress. Its how most of the record speed ascents have been set, with variations to the basic method I'm sure.

that's assuming that the second can jug the rope faster than the leader can climb? otherwise the leader ends up stuck mid-pitch waiting for the second to arrive and unfix the rope?

Jeff nailed the essence of it: after climbing a pitch, rather than belay up the follower, the leader pulls up the rope and fixes it. He then begins to lead the next pitch while the follower jumars and cleans.

You're right that the follower must move quickly (relative to the leader) for the setup to be efficient. Short fixing is most often employed on aid routes, where the leader is likely to be slow.

I wrote a little article and how-to section on my blog about simul-climbing and shortfixing, check it out here:
ropeandsummit.wordpress.com…

Stay safe!
Scott