"Short-tracking" is the horribly mis-named short-hand name for the ABIM research pathway. It allows one to enter a fellowship after the PGY2 year. The only thing short about it is the amount of time spent in IM, otherwise, it is a longer path to sub-specialty certification. As an example, I'm doing this path in IM/Onc. If I did just IM to Med Onc (no Heme training, which would add another 6 mos), it would be 5 yrs total. My path is 6 years although 3 of them are FT research. Nobody who does it calls it short-tracking.
As for when "most" people start, that varies. The traditional course is to apply as a PGY2 and start after finishing residency. But with the rise of the hospitalist movement, its become more common to apply later and do a year (or 3) as a hospitalist.
Finally, 2 pieces of advice...stop asking the same question in multiple threads across different forums (your UK internship question) and decide whether you want to do GS or I'M. If you're not "all in" on one or the other, you're likely to wind up with neither in the end.