Fast-tracking in IM is typically done by those who plan a research/academic career and can justify leaving an IM residency after two years by planning to go a lab for a few years in as part of or as a prelude to a fellowship. Where I train, one of my residents this year is fast tracking into ID at Baylor in Houston -- so he'll finish his second year of IM this year and then "fast-track" into fellowship -- however, he is spending two years in research work before doing the clinical portion of his training, what would have been 3 years IM and 2 years fellowship is now 2 years IM and 4 years fellowship. 6 years either way.
Another guy in my program is fast-tracking into an Endocrine fellowship at the Brigham. He has funding to set up his own lab there, so who knows how long he'll be in the research portion of his fellowship before he does the clinical part.
One of my co-interns plans to do Cards, and has a lab waiting for him at the end of next year. He plans 2-3 years of research and will apply to Cards fellowship during one of those years. The lab he's going to is indirectly related (maybe) to Cardiology, but he loves basic science.
So bottom line, fast-tracking is allowed to so that you can move on to the next phase of training a year early because you're gonna be spending more time in post residency (fellowship with significant research time) then if you did the traditional route.
Also, if you notice above, the people fast tracking are going in to research heavy non-competitive fellowships. My friend going into Cards does not have his fellowship yet, but will be applying for it during his research tenure.
As a general rule, because of the competitive nature of Cards and GI, people fast-tracking into them is pretty rare. There are always the people who have 3 PhDs in cardiovascular physiology or in liver metabolism that can say with credibility that they will be pursuing an academic/research career who may be able to justify and make such an arrangement, but that's obviously pretty rare.
Hope this helps.