This gear is a bit of a spoiler in its own right, so please forgive us for being just a bit vague in our description. ^_^
Weltall 2 is basically an "improved" version of Fei's gear. Taura repaired it after the incident at the end of disc one, and installed several interesting new features that were designed to draw on Fei's latent power -- or to enhance it. This is, if you get technical, a vehicle that belongs in the Omnigear class, but Xenogears is really the one that fits that description.
The "special" system, of course, is the greatest of the improvements Taura made. It enables Fei to go immediately to Infinity Mode, and remain so for the given three turns before he reverts to normal. IT does, however, cost quite a bit of fuel (1000, I believe), so its usefulness depends on how desperate you are. The nature of the system is a spoiler, of course, though anyone past this point in the game should know what it is -- no need for me to explain it!
Weltall is a reference.