I'm pretty sure it is where the fluxcapacitor used to go. Because fuel for these became hard to come by, and complications stemming from their use, most were ultimately removed. Sadly, no original units are known to exist, and government regulation due to international treaties has effectively forestalled attempts to provide an after market version. I have attached a rare archival image of some of the design team members during testing. This item was useful in providing extra boost on long grades, but had to be used with care.