I'm not exactly sure. I've only done it once, and when I thought I was in transition, the doc measured and said I was only at 4 cm...it was kind of weird. I went through two phases: 1) I started having VERY strong contractions, fairly close together with an urge to vomit, though I just retched slightly and never actually vomited. I remember thinking "I get it now! This is why women say they have the urge to vomit!" because it felt kind of like a whole body puke - like the urge started in my cervix and worked its way up. It was weird, but I was stoked because I was positive I was in transition. 2) Here's where things got bad - I started having contractions right on top of each other with no breaks. And I mean no breaks. Not even the short 30-second breaks I've heard we're supposed to get in transition.
: AND I was having an uncontrollable pushing urge. This happened just after the vomity phase passed and the doc said I was only dilated to 4. So of course they told me not to push, which was the most godawful pain ever. I never, ever want to go through NOT being allowed to push when my body is telling me it's doing it anyway.
So! Maybe it was a two-stage transition, but that second part was the pits. It only lasted 30 minutes, fortunately.