Oh, i see what you're saying. Yes, pretty universally, an internship is the year after your graduation. In the US, most programs also consider the intern year as the 1st year of residency but you still have the option to explore another field after your intern year (sometimes requiring you to do another intern year). This is before you become committed to an actual residency.
I think theoretically, an IMG with no US experience can still apply for the match and thus internship, if he/she completes the USMLE Step I and in some cases, Step 2 CK and CS as well. However, chances are much increased if you do complete US electives because residency directors will know of your experience, and they are ultimately the ones who decide to place you in their programs and allow you to begin internship (and beyond) there. In otherwords, the US electives/training should be completed before your internship, and thus before you graduate with your degree. As a matter of fact, most schools that I know of will not let you complete an elective if you have graduated from medical school. An externship is also something that should be persued before you graduate, if such an opportunity presents itself.
So a quick answer to your question is that an IMG who wants US experience should try to do it before he/she graduates, but an IMG graduate who has no experience can try to match for internship/residency all the same, but not having US experience may close some doors. Having said that, some are successful, but oftentimes these are people with significant experience in their home countries, who have already been interns/senior doctors in clinical practice, and they may be required to repeat internship or in fact, the whole residency.
I think you could do an observership after you graduate, but I agree with Kim that in terms of your academic career, observerships are pretty useless.
