It’s like this. They divide the total racers in a class into groups after qualifying. The 1st qualifer is 1st in group A, the 2nd qualifer is 1st in group B and so on till all groups are filled. You have as many groups as it takes to divide the field into groups no bigger than 6 to 10 but always an even number of groups.
You then have as many pre-finals as it takes for each group to race every other group. If there were 4 groups, then there are 3 per-finals for each racer. A vs B and C vs D. Then A vs C and B vs D. Then A vs D and B vs C. You get points based on your finishing postion in each pre-final. The lowest 20 go straight to the main event. The next 32 go to an LCQ. The rest go home. The top 12 in the LCQ go to the back of the main and we crown a chumpion,…er champion.
It’s a good way of getting a lot of racing and handling a big field. In the old days when the SuperNats really drew big fields, just making the main without going through the LCQ was a big deal.