Sixes aren't "rigged" or "fixed" but they are, uh, "choreographed." Typically, for sprinters, each will be offered an opportunity to shine (in a keirin or similar), but the sprints will be pretty straight.
In the enduro events there will often be a general script - some lesser races will be essentially neutral and offered up to lesser teams for opportunities to get at it (take a lap, take the win, etc); the bigger closing chases (e.g., madisons run by classic rules where points are just tiebreakers for teams that are on the same lap - so it's all about taking laps) are wide-open/flat out and the big teams come to the fore. organizers often try to keep things so that the six comes down to the last race of the last day and so that there's a local favorite team vying for the podium.