Can someone please explain how this mechanic works?
Rondel - how does this work?
A rondel is a circular chart of actions that happen progressively. It looks sort of like a pie chart with each area of the rondel denoting an action (move/build/reinforce/buy, etc). A player's piece is moved around the rondel via some other mechanic (usually progressively, with the possibility of moving more than just one space over due to some special situation). This allows you to have an idea of what actions you may take in the coming turns and what other players will be able to do (and cannot do).
Refer to this image:
http://www.boardgamegeek.com/image/250870/imperial
Note that it states that the player can move up to 3 spaces around the rondel clockwise for free. I believe in imperial you can pay to move to the 4th space? I forget, havent played in a while.
I think if you pay you can go how far you want to. But the normal thing is to pay for the 1 extra move in the end and just step on the two sections that gives you money. The pay for an extra step is very rarly used in the games I have played, exept in the endgame.
In Imperial you can pay to move up to 3 additional spaces, so lots of options but you cannot quite buy an entire "lap" in a single turn.